PS4 has slightly better hardware, but the actual difference to your eye or in the games is mostly imperceptible.
I would not put too much stock into player prices. They were largely broken by the illegitimate Fifa 16 coins boosting. It won’t necessarily always. be cheaper prices on Xbox.
I also would not worry about population bases. PS4 currently has the lead, but we are very early in this generation still. Also, both consoles have a huge population on FIFA, and it’s not like this game ever has issue with not enough people being online.
The Xbox is better as an all around home media device, especially if you get the kinect. Most gamers don’t base their console decision on home media features, though.
The only other factor, in your case, since you don’t care about contollers, is the exclusives. If you plan on playing other games, check out the exclusives lineups of each console and decide from that.
Go for the exclusive games you prefer more. I played Fifa 15 on pc this year but im getting a Xbox one because of Fifa 16 (and legends) and for the new Halo. But beside the exclusive games, they are pretty much equal. But if you dont care about legends or other Xbox games, i would go for PS because the playerbase is bigger.
I have both, and FIFA 15 on both. I dont know that either is really better than the other, and I am struggling on which to choose for ’16. I am leaning PS4 since I am in the US and there are less FIFA players per capita and its simply a numbers game. More PS4s right now means more people that also probably play FIFA on it.