Few different reasons that added up for me
1)much prefer Xbox controller over ps4 for games I play more then a few hrs at a time. My set up doesn't like me plug in so when battery dies I'm force to stop, and yes this happens a lot to me. Xbox lets me swap bats instantly.
2) hdd space, much easier to add more to Xbox. I tend to keep all my games installed. Only got 1tb on ps4 and have easy assess up to 5tb on my Xbox just sitting around my house.
3) Direct X 12 patch?
4) had Witcher 2 on Xbox 360, kinda like keeping series on same system family (ie all assassin creed, battle field, far cry I buy are on Sony systems while all dragon age, mass effects, cod end up on Xbox systems). Yeah this is the dumbest reason.
If it was a huge difference I'd go ps4 but unless something changes in next few days it will be Xbox for me.