Battle of the Black Dive Watches: Sinn U1 S vs. Seiko Prospex SLA035 MM300 Watch Review (1010.020 vs. Marinemaster 300 SLA035/SBDX033)

Last year, I wrote a tongue-in-cheek piece declaring the Sinn U1 S "The Best Diving Watch Available." If I'm being serious, I'm not remotely qualified to evaluate anything related to diving. I have reviewed a lot of watches though, and I believe that the qualities that make it a great diver also make the U1 a great watch for everyday use.

That being said, the Seiko brand is no stranger to building dive watches. As luck would have it, I recently picked up a Seiko diver that, on paper, is very similar to the Sinn. Nicknamed the "MM300," these Seiko models are nearly equal in dimensions, specifications, and price to the U1. My all-black-with-red-accents SBDX033 (a.k.a. SLA035) is also visually very similar to my all-black-with-red-accents S version of the U1.

Hopefully, you've already read the reviews of each in the links above and know what they are all about individually. With that out of the way, how do they compare?

Comparing Sinn vs. Seiko

The watches will be compared in a number of categories, and then total scores out of ten will be added up at the end. Because I have not taken either watch diving, this comparison will rank the qualities of both watches for everyday use, as well as some easy to quantify specifications that don't require entering deep water to evaluate. I think this will be of value to most readers, but if you have actually taken either model diving, I'd love to hear about your experience in the comments.

Please note that I do not have an OEM strap for the Sinn.  Thus, I have chosen a few of our most popular black rubber straps to display both watches on a similar footing. These straps can be purchased via the buy buttons in the article.

Now, on to the review!

Wearing Comfort

If you skip to the end of this article and read the specs, you'll find that both watches are within a few tenths of a mm in all dimensions, and also within a few grams of weight. We're splitting hairs here, but these differences lead to some marginal differences in comfort.

Both watches wear surprisingly well for 44mm dive watches that are 14 to 15mm thick. They both have low-mounted lugs that curve down, and that are fairly compact to the case. A lug-to-lug distance of around 50mm is fairly compact for such big watches. The image below shows that the Sinn is slightly taller but by only 0.2mm.

Finally, the Sinn does take a narrow margin in wearing comfort. It has the feeling of its center of gravity being closer to your wrist, partially due to the fact that it is thinner, and also a few grams lighter. The Seiko can feel a little top-heavy, especially if you don't wear it on just the right strap.

Additionally, while the Seiko's 20mm lug width has the benefit of making the watch look smaller, the 22mm straps that fit the Sinn help keep it more secure on the wrist.

Other than those small differences, they are similarly comfortable. Both have their crowns offset at 4 o'clock, so there are no issues with the crowns contacting your wrist or any other sources of discomfort.

Bezel

Though their bezels are quite differently shaped, both are very easy to grip and turn. The Sinn bezel hangs over the edge of the case slightly, while the Seiko has a tall, tapered bezel that provides plenty of finger space.

Beyond this, functionally as a tool watch, the Sinn's bezel works slightly better. Its chunky, 60 clicks are easier to feel than the Seiko's 120. The Sinn bezel does have a bit of lateral play that doesn't exist on the Seiko. Additionally, it is a captive bezel, which means that screws must be removed before it can be taken off the case. In theory, the Seiko's bezel could get knocked off by an impact, or its ceramic insert could be cracked.

But for the way that I use a watch, these things are less of a concern. I like the quality appearance of the ceramic (complete with luminous paint) on the Seiko and the fluidity of the bezel. Even if it's less robust, it feels like it was more expensive to make. The ceramic should also be able to better withstand scratches from minor impacts.

Legibility

The Sinn is the clear winner in this category (no pun intended). Although the Seiko has good legibility, the U1 has the best legibility of any watch that I've ever owned or reviewed. The large, flat white hands contrast very well against the expansive dial in bright light and shadows. Red centers help to visually orient the hands, and the hour markers pushed to the perimeter are also very clear. To top it off the flat crystal sits close to the dial (impressive for a watch with such water resistance) and has amazing AR.

The Seiko has a good dial and hand layout, but the dial is smaller than the Sinn's. It also sits in a deep recess within the watch. Its hands are shorter and narrower, and the silver doesn't pop against the dial as pure white does. Additionally, while the Seiko's AR is fairly effective in reducing glare, it doesn't completely eliminate it as the Sinn's does as it is only applied to the underside. This can be seen in the photos too.

In the dark, the Seiko's lume is significantly brighter (the brightest of any watch that I can recall having owned or reviewed), but it has less surface area due to the size of the hands. The fact that there isn't even a frame around the lume helps the Sinn still put up a fight well when the lights go out.

Case Design and Finishing

Between its tegimented steel, and matte black coating, the Sinn is designed to have a finish that will stand up to abuse. This functional application earned it some good points in the finishing category.

That being said, the Seiko appears to have significantly more effort put into its case finishing. The Sinn has one single bead-blasted finish that was likely applied across the entire case by a machine. Seiko, on the other hand, uses its zaratsu hand-finishing along with precise brushing to give the case a more luxurious appearance. The Seiko picks up more fingerprints, but it looks more dynamic as light is reflected off of the case, bezel, and also the polished hour markers and hand edges.

The Seiko also made up some ground on the Sinn due to its monocoque case design. Sure, it provides no practical benefit for non-divers, but I love when watch companies put additional effort into technical upgrades like this.

Appearance

This is the most subjective portion of the review, but also one of those most difficult as I find both watches very attractive. While the Seiko has a more expensive, jewelry-like appeal, the Sinn just looks cool! I also find its hand and dial designs more attractive. The Sinn is so functional that it looks good. Finally, this category is a draw.

Strap

While I didn't have the factory Sinn strap for this review, I do have experience with one from the Sinn U1 Camouflage review that I did last year. Compared to the Seiko, it is a much nicer piece. The silicone is thick and has a high-quality feel.

Also, the deployant clasp with dive extension is extremely nice, especially compared to the Seiko's tang buckle.

The Sinn's strap is also curved to fit the case. This, plus the elimination of a loose strap tail gives it a cleaner look and a more seamless wearing experience. The Seiko's soft silicone strap end tends to fall out of its metal keeper.

The Sinn strap does lose a few points because it needs to be cut to fit, eliminating the chance to adjust it later.

It's also worth noting that Sinn does offer an all-black tegimented bracelet option for an extra cost. Seiko doesn't have such an offering for the black versions of the MM300, so I've focused on the straps for this review.

Water Resistance

Despite its more conventional case design with a removable caseback, the Sinn is able to handle 1000m vs. the Seiko's 300m. Sure, 300m is well beyond what any human being will ever need, but the Seiko is not any thinner than the Sinn as a result of its lower capacity. Sinn must build its cases and gaskets to higher tolerances.

For any saturation divers that might be reading, neither watch uses a helium escape valve yet both are can be used for saturation diving. Seiko accomplishes this through its monocoque case that resists helium entering. Sinn secures its crystal so well that it can't be forced out when the pressure inside the watch is higher than outside (also potentially useful for pilots if cabin pressure is suddenly lost). 

Movement

The Seiko recovers a few points in this area. Both watches have the same beat rate (28,800) and jewel count (26). While the Sinn's Sellita SW200-1 gains a bit by having instant date change, the Seiko's extra power reserve (50 hours vs. 38) is a more practical upgrade.

Sinn doesn't list the accuracy of the U1, but even the highest (non-COSC) grade SW200-1 is not listed to be as accurate as the Seiko. Both of my particular watches run at a much higher accuracy than their 10 to 15 second per day ratings anyway.

The Seiko also gets an edge for the cool factor, and additional development cost of having a fully in-house movement.

Value for Money (Used Price)

Looking at MSRP only, these watches were again within spitting distance of equal when new. As far as I could find though, the run of 600 SLA035s released in 2020 is sold out. Also, at least at the time of writing this, the U1 S is sold out at the main U.S. distributor.

Because this means that most people will likely be buying these watches second-hand, I've compared their used value (taken from data collected by our friends at WatchCharts.com at the time of this writing). With the Sinn being almost $1,000 less expensive, it gets the win in this category.

If you were to find one new, you could score the watches equally here. The Seiko would also get the win if we had a value retention category. This is likely due to its specifically limited production, although the S model of the U1 is not easy to find either.

Overall

To be honest, choosing a winner between these two is causing problems for me. Because they are so similar, I told myself that I'd put my money where my mouth is, and sell whatever one that I liked less. I'm struggling with that decision.

Your thoughts?

Which black dive watch do you prefer out of the two? Which one should I sell, or do I need to keep both? Maybe I should get the green MM300? We'd love to hear from you in the comments!

Specifications