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.
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.
If games like Knack and DriveClub can sell 1 to 2 million copies than anything is possible.
Never underesitmate the power of forced bundling.
Where i am from UC4 was bundled in majority of PS4s, you could not buy a PS4 without UC4 at xmas time unless you brought a Pro. Funny how its sales went from 4m to 8m so fast. Any company can do that, MS could have done it with H5 however there happy with its $400m profit the game turned in. Thats bigger than most movies. Go find out how much money UC4 or Horizon made for PS?
Ill wait right here.