Red Dead. I've played both and Red Dead is by far the better game of the two in my opinion. The only thing God of War does really good that Red Dead is kinda lacking in is how the game makes use of the boy as your partner, and the impact that stuff has on your relationship with the boy. In Red Dead the interaction is there in dialogue and cutscenes, but outside of that you're just riding with them or shooting with them, and it's always someone else you're on a mission with. Outside of that Red Dead 2 kinda outshines GoW in every way. I love God of War, and it was my favorite game of the year until Red Dead came along, but I wouldn't put God of War in my top 3 games of the generation, and right now Red Dead 2 is sitting at the number one spot.
They are both amazing games, but Red Dead 2 really took games to the next level for me.