I had the Gear S3 Frontier and upgraded to the Galaxy Watch. Basically, the Frontier has MST for Samsung Pay. The Galaxy Watch has 5 ATM water resistance (i.e. you can legitimately swim with it, unlike the Gear S3), and the battery life is far superior.
Swim-ready water resistance is great, but for me the real tipping point is battery life. Since I wear my watch all day and also use it for sleep tracking, it has to last well over a day. The Gear S3 was advertised as having a 3 day battery life, but after various software updates it barely made it a day. Plus, going out for a 30 minute jog with LTE and music streaming via Spotify gave me a 40% hit to the battery life. So basically I'd never be able to go for more than a full hour walk or run with the thing. The same level of usage with the Galaxy Watch only takes it down by 15%. So I'd say the GW is well worth the higher price, even with the loss of MST. After all, if you have a Samsung phone you've got MST, and how often are you really going to have your watch but not your phone, except when working out? And of those times, when are you going to make a purchase?
Having said that, I will say that I do sometimes stop at a gas station on my jogging route to pick up a Powerade. I guess I'm lucky they take NFC.