Speaking as a 360 owner, if I were coming to the decision cold I’d be leaning towards a PS4. The one thing that would convince me to stay on Team Green is if all my mates ended up with XBones instead. That or if they announced a follow-up to Red Dead Redemption as a platform exclusive.
But. And this is a big “but” (I like them and I cannot lie), there is little compelling reason for me to upgrade at the moment. Most new releases come out for all four platforms (360, One, PS3, PS4) and the back catalogue for the whatever-gen-we’re-calling-them-now older consoles is comparatively huge. An Xbox One or PS4 with maybe a second controller and a few games will set you back ~£500; a 360 or PS3 bundle will be half that, and you could get a dozen top-notch games with the remaining £250.
As an investment in modern gaming, and a shiny 40th present to yourself, the PS4 is probably your best spend. As your first console in maybe fifteen years, right now I’d be seriously considering a 360 and a Gold subscription.