Easily Xbox. And as some has said, it isn't close. Taking into account exclusives released and scheduled to release (Forza games, Gears 4 & 5, Halo MCC, Halo 5, Infinite, CD3, Sea of Thieves, etc) and the fact that XBL has wildly better features (party chat is top notch, LFG, clubs, scheduled competitions) than PSN and I'd be hard pressed to find a reason someone would think PS4 is better for competitive multiplayer. Controller preference would really be the only thing.
Literally all of the PS4 exclusives that get dropped in other threads for reason why PS4 is the better value or has better exclusives are single player experiences. And that doesn't seem to be changing in the next 1.5 years