Which ever one has the games you want to play.
At launch, neither
If I buy another console it'll probably be an Xbox, simply because I prefer their controllers. They're ergonomically designed and the contours fit your hands, whereas Playstation controllers are all hard lines, ultimately making them less comfortable to hold onto for any length of time.
Was there a claim that you can't buy pre-owned games for the Xbox 1,but you still can for the PS4 :dunno:
At launch, neither
It's changed now. You can now play used games on Xbox One, just like the PS4. Microsoft changed course after most gamers put up a fight, aka their sales were probably affected.