Xbox One at the moment. I am playing Watchdogs on the PS4 but Plants v Zombies, Forza 5 & Dead Rising 3 Co-op on the Xbone.
The Xbone is also used more as a media machine as well (Youtube, Twitch etc).
What somewhat muddies the waters is I have two Xbones (for co-op) and only one PS4 so it isn't really a fair fight
Multi plats where there is an obvious visual or performance difference I will get on the PS4 (BF4 for example), everything else I will get on the Xbone as I prefer the controller.