I have both a PS4 (bought at launch) and a One X (upgraded from a base XBO bought in Nov. 2015). I own both systems because both PlayStation and Xbox have enough first-party and exclusive games to make them worth owning.
The PS4 was initially my primary system as most games played better on it than on the base XBO. But once the One X was released, I started buying all my third-party games for it for the same reason: better performance (I also honestly prefer the XBO controller over the DS4, though the DS4 is admittedly a huge improvement over previous PS controllers). I even double-dipped a few titles just for the 4K upgrade.