In my opinion, there's no reason to buy an Xbox. Its cool that it's going to have backwards compatibility but considering how little of it we've seen, I think its going to be badly implemented like it was on the 360 with the original Xbox compatibility. One day they just quietly stopped adding any new games and a bunch of the ones on there had weird problems. Also, I already have a 360 to play those games on, and 360s are so incredibly cheap to pick up now, this isn't really a selling point. Though the 360 just sits there gathering dust, I'll occasionally play a shmup or whatever that only came out on it but that's all.
None of the exclusives interest me, and a lot of them are going to come out on P.C., or will at some point (see Killer Instinct for that, though I still have no desire to play that and I'm a big fighting game player). It's cool that they're trying to make it do stuff with PCs, but why not just get a PC if you want all that stuff?
All of us here are into AC obviously, which to me is a very "Japanese" style game, as are all things Fromsoft make. There's every chance them and companies like them won't release their stuff on PC, but they will on PS4 because the Xbox is literally non-existant in Japan right now (they sell about 200 a month across the whole country, or some stupid low number). These are the kinds of games I like, so for me the PS4 was the obvious choice. I may get a gaming PC at some point down the line but there's nothing on there that makes me really want one, and most games that are out on both end up being worse on the PC because they're poorly optimised (see Batman).
So to me, PS4 and maybe a PC covers everything I'm interested in. The Xbox seems irrelevant at this stage.