No Man's Sky is also out on PC so I'm going with NMS. It really does look unique.

Horizon has more mainstream appeal.
No Man's sky is viewed as a very expensive indie by the mainstream. It's out on PC too, yet I bet there will be many people waiting for a sale.

I feel like Horizon will end up he bigger seller, at least 1st week. Though I feel it could be LT as well.

Neither will sell nearly as much as the hype would want them to but it's impossible to know which will sell better without knowing if either of them are going to be good games or review well.

I would say NMS since it's on two platforms and I feel there's more buzz around it, at least at the moment.

Week one? Definitely Horizon

i'm only interested in Horizon Zero Dawn
but No Man's Sky will have chance to outsell it

No Man's Sky, easily. Will be huge on Steam. Tons of people in my Teamspeak community plan to buy it. They usually laugh at spending $60 on a game the day it comes out. But this has some hype.

I'm going with Horizon for first week. However, NMS should be a slow burner that just continues to sell over time. A bit like Minecraft, but of course on a much lower scale. I also think the 60$ price tag is just a marketing strategy. I'm quite certain there will be a price drop soon after launch to get massive amounts of people to get the game. If it was 30 $ from the beginning, everyone would have been cautious about to get the game or not. But if it's at 50 % discount for a limited time 4 weeks after launch or so, many people will get the game because they think they made a real bargain. It's a quite old marketing trick.

If we're including the PC version with No Man's Sky then I think it will most likely beat Horizon lifetime and possibly first week as well. I'll still go with Horizon winning FW mostly because I expect it to release during the Holiday season.