I think whichever one I buy I will see exclusive games on the other that make me wish I bought that one.
But looking back now that I think about it... if it's like my playstation history I would get the new one when a GTA game came out that kinda made me. GTA 3 made me buy a Ps2, and GTA4 made me buy a Ps3 (okay once that first UFC game came out I would have anyway but that's what it was). But GTA5 isn't even going to be a Ps4 game I heard so not selling my ps3 anytime soon. But if the ah, ps4 wasn't backwards comp. I wouldn't sell it anyway.