Very little would convince me to take an Xbox one over a PS4 right now. It's not about the money for me, I just prefer Sony's first-party output and have zero interest in shooters, racing games, Kinect, multimedia stuff (my tv can do that already), online gaming in general and Fable, so I just don't gain anything by choosing an Xbox instead. Even if it was £100-£150 cheaper I'd rather pay more to get access to the games I want, and if I went for the cheaper option I'd probably regret making a decision based on price rather than the games range when it's a hobby I spend hundreds of hours a year on.
I also don't have space for both under the telly, but it's super-cheap in four or five years time and they have half a dozen great RPGs or platformers that I can't play anywhere else, then I might pick one up and swap it in for a few months when my PS Plus sub runs out, something that also heavily supports my Vita and another reason I'd rather have a PS4. I'm absolutely not paying for both XBL and PS+ at the same time, that sounds like madness.
To be fair, I'm not even considering a PS4 until this time next year at the earliest, as while I'm far more interested in it than the Xbox, I can wait for a hardware revision, heavy price cuts and more games for both machines before making a decision. Right now neither looks like an attractive replacement for my PS3.