Ultimately go where your friends are.
Outside of that, its usually where there are exclusives, if any. PS4 have bossed it in for the last good few years with all the amazing exclusives they have. Sort of like when Xbox 360 was king with all the exclusives versus the PS3. It also didn't help the Xbox One on launch lost all the awesome UI features and ease from the Xbox 360 when it launched but I appreciate its started to become better in the last 6 to 12 months.
Exclusive wise - God Of War, Horizon Zero Dawn, Spiderman, Last of Us...they've just killed Xbox with them over the past few years.