God of War easily.
I can see Sea of Thieves becoming pretty popular though.

Well ill put my money on GoW however that's not based off the game being better but with afew key factors that make these type of comparisons hard and pointless.

1st - GoW will most likely be heavily bundled with most PS4s which will mean the game will sell a ton loads regardless of its quality. Like UC4 and Horizon it will have massive sales just based off the PS4s high demend. Some companies put more efforts in bundling than others so this is a big point to take into consideration.

2nd - Digital sales is becoming bigger and bigger every year so we wont ever know how many these games do digitally, some sell better than others. Games like Sea Of thieves will probably be more popular on the digital side based off its always online game genre.

3rd - Xbox Game Pass will take away from the physical sales which will make game comparisons handicapped.

4th - New IP focused on a completely different genre compared to an already established IP with a lot more marketing behind it.

Game comparisons this day and age is pointless since there isn't an accurate figure of digital sales and game passes plus games being bundled more than other games all make for nothing than just a one sided slag fest.

This is actually a very good point. The most pertinent question then, would be, 'Which game will make more profit (or revenue, if that's your thing)?'

I dont think thats very good either. As these are both first party titles and first party titles tend to push more for hardware move than profit. Its why they tend to be lacking microtransactions, wich is how games now days make the bulk of the cash. I do believe sea of thieves has some micro transactions, don't remember what exactly, but by default then sea of thieves should come on top due on revenue. 

I actually agree and iv been debating this topic for awhile now, Games should be looked at as which game makes more profit/revenue because game sales are no longer accurate and can be hugely unfairly viewed upon especially if one game is offered as a free game via a pass and the other isn't. Modern gaming is a lot different to compare software sales in a digital strong world, not like the Nintendo vs Sega days where you could only buy the physical games and bundles were extremely rare. 

God of War

Yes we already now SoT will be better, MS doesn't do heavy bundling, that customers are obligated to chose the bundle of GoW and not any other available... and of course Horizon ZD had zero chance of doing good sales since it was a new IP, owww wait, it outsold Halo 5 which is from a very well establishedfranchise.

So, in short, you're betting on SoT to do better?

Is this a serious question? One is a new IP on a user base half the size of the other, which is getting a new release in a major series. God of War will outsell SoT by a mile.

I suppose it is theoretically possible that SoT could get amazing reviews, and ride its streaming success into mega-sales. GoW could, in theory, get shit reviews, release along side a bunch of much better games, and struggle. So, in this theoretical case, it may be possible for Sea of Thieves to be the bigger seller. This is so unlikely as to be totally ridiculous though.

I'll go with God of War.