You are opening one of the most heated debates: PC vs Consoles
Obviously PC. If you are on a tight budget the upcoming Steam sales will boost your games library. In the long run you can always upgrade your PC, add RAM, change GPU, etc etc. This will give you the best for your money, as you won't have to throw things away like PSU, case, peripherals.
The XBox is an "one time off". Once new games stop coming out -end of the this years I suspect- you will have to throw it away and get a PS4/XBOne or a PC.
Oh do not forget: If you are buying the PC from a system builder or eBay, have a look at building your own machine. You may end up with better components at a similar or better price. Either way get a PC!