Measurements

I think the dimensions of the two watches are the first thing you should consider when having a comparison. The Vostok Amphibia is clearly the biggest/bulkiest of the two since the case diameter and lug-to-lug length are 3mm bigger than the Seiko 5.

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If your wrists measure anything below 6.25", I think you should avoid the Amphibia as you will almost certainly have lugs overhang. Conversely, men with large wrists (7" +) might want to steer clear from the SNK809 as it might look underwhelming.

Another important measurement to factor in is the thickness of the case, which is a massive 14mm on the Vostok. This means the Amphibia cannot be worn in a formal setting, as there's no way the huge case will slide under your cuffs.

On the other hand, the thin profile of the Seiko 5 makes it perfect for casual and formal wear, as it slides under cuffs without a problem.

Finally, the lug width of 18mm is the same for both the SNK809 and the round case Amphibia. Be careful though, because some Amphibia models are equipped with the square case (as pictured above), which has 22mm of lug width instead.

Specs

Crystal

The crystal of these two watches differs both in material and shape. The Amphibia uses a slightly domed plexiglass crystal, while the Seiko 5 uses a flat Hardlex (mineral) crystal.

The plexiglass used on the Vostok is the most shock-resistant of the two (it is basically unbreakable), but it is super easy to scratch. On the other hand, the Seiko 5's crystal is a bit more prone to shattering, but it will be much harder to scratch.

Water Resistance

The water resistance is also a big differentiating point between the Vostok Amphibia and the Seiko SNK809. The Russian timepiece uses a screw-down crown and upgraded seals/gaskets to reach a very respectable 200m water resistance.

That's how this model earned its name, as Amphibia is a loose translation of "diver's watch". In contrast, the Seiko 5 uses a push-pull crown, which only allows for 30m of water resistance. This means you can't take your SNK809 for a swim without risking leakage.

Case Back

As you might've noticed by now, the Amphibia and Seiko 5 are two very different watches, and the differences continue with the  case back. Vostok opts for a stainless-steel case back, while the Seiko has an exhibition case back that lets you peek through the movement.

Lume

The luminous phosphorescent is one of the areas where the two watches resemble each other. On both timepieces, the hands are coated with a lume application. Also, they both have small lume pips at the extremity of every hour mark.

Seiko's Lumibrite application is just a bit brighter and more durable than Vostok's application, but both are very functional.

Movement

While both the Amphibia and the SNK809 have an automatic in-house caliber, the Seiko 7s26 is the better one. The major difference between the two is the power reserve, which lasts 9 hours longer on the Seiko.

Also, the Vostok beats at a very low frequency of 19,800 bph, compared to Seiko's 21,600 bph. The Vostok does have more jewels than the 7s26, but that's more of a gimmick than an actual benefit.

As for the accuracy, both watches will have "good, not great" timekeeping. In actuality, you can expect the precision to range from -10 to + 10 seconds per day for the two watches.

The only place where Vostok might one-up Seiko is in the shock-resistance department, as the 2415/16 movement is basically bomb-proof. 

Design

Styling/History

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Bezel

With so many variations available, it's hard to speak on the Amphibia's bezel design as there are too many to choose from. The most popular one is the 60 minutes bezel, which features Arabic numerals every 10 minutes.

The two other most popular models are either the 60 minutes bezel with Arabic numerals every 5 minutes, or the one that features 12 dots for the hour marks (often used on Scuba Dude editions).

On the other hand, the Seiko SNK line is only available with a fixed bezel, that has a brushed finish.

Hour Marks

Once again, the Amphibia is offered with tons of different dial layouts. The most common occurrence is a clean and simple dial with large Arabic numerals for the 3/6/9/12 marks. Other choices include smaller Arabic numerals for every hour mark, or applied-indices only, as is the case for the Scuba Dude edition.

The Seiko SNK809 also uses Arabic numerals to tell the time. On this model, the outer ring of the dial features numerals every 5 minutes, whereas the inner ring of the dial has numerals for every hour. This makes for a very easy to read dial. Also, the red paint on the tip of the seconds hand enhances legibility even more.

Hands

For once, the set of hands used for the Vostok Amphibia is the same across the whole line. The hours hand has a thick arrow shape, the minutes hand has a simple sword design, and the seconds hand has a simple needle shape.

The Seiko SNK809's hands are also quite peculiar: some cool-looking lozenge hands for the minutes and hours, and a red-tipped needle seconds hand. 

Straps/Bands

Writing a section about straps and bands for these two watches is basically pointless, as you will most certainly want to change the OEM strap for something third-party as soon as you get your piece.

Both Vostok and Seiko make pretty bad straps when it comes to entry-level watches, but I think the Seiko 5's strap is just a bit better. The SNK 809 is offered with nothing but a black nylon strap, that is stiff at first but gets more comfortable with time.

On the other hand, the Vostok Amphibia can be bought with anything from a nylon strap, to a leather band, to a stainless steel bracelet. None of these do a great job at what they're supposed to, so I strongly recommend getting a high-quality strap or band.

Value

Pricing

As you can see from my estimated price range above, the mean price for the two watches is the same, but the SNK809 has a larger distribution. This is simply because the SNK809 isn't officially sold by Seiko anymore, so it is affected a lot by supply & demand.

The SNK's price seems to dip right around Christmas (from thanksgiving to new year), and it often goes up for the rest of the year with a few exceptions.

Conversely, the Amphibia is still mass-produced by Vostok, so the price is much steadier. You can get most examples for a very reasonable cost, but some limited editions or vintage models will run you well over $100.

Warranty

The manufacturer's warranty available with both watches will vary depending on where you're situated in the world. Vostok offers 2-years warranty for anyone in Europe, whereas U.S customers will only have one year.

It's important to note that you'll have to send your watch to Russia to get it repaired, so you will have to spend 15-20$ for shipping, plus a few weeks (and sometimes months) for the repair.

According to Seiko USA's website, every Seiko sold in the U.S is offered with a 3-years manufacturer's warranty, whereas watches sold anywhere else in the world have a 1-year warranty.

Brand Recognition

I've given a low score to Vostok's brand recognition because it will only get recognized by watch enthusiasts or actual Russians. Seiko enjoys a slightly better reputation in the U.S, as some of the general population will recognize the Japanese brand. Of course, none of these brands are as recognizable as Timex or Swatch.