From MY point of view,
XBox advantages:
More of my friends are on XBL and will buy the XBO.
I prefer the user interface.
I much prefer XBL to PSN.
The controller.
I find the exclusives better to play (Forza Motorsport 5) - this may be due to more friends playing, as said above.
Achievements > Trophies.
Kinect 2 included.
TV integration etc.
PS4 advantages:
Price.
Used game compatibility.
Region free.
The exclusives.
It's all very subjective, and I will be buying both consoles, but if a game is multi-platform, I'll always tend to go for the MS version over Sony. Just my preference because of the advantages I listed from my perspective.
Both will be superb consoles and I'm really looking forward to seeing them in action.