I never understood the appeal of the Instant Game Collection, nor this Games with Gold business. If I want to play a game, I buy it. The games offered through PS+ are at least 6 months old. The chances I do not own a game 6 months after release that I want to play, are really small.
It never happened so far. PS+ has offered me exactly 0 value and I have no doubt Xbox Live will be the same. Unless they start offering free games at launch or at least no more than a month old, there is no point to it for me.
Especially since you just rent them. On the off chance I do want to play a game I did not yet play on PS+, I'm not going to pay 50 bucks just to rent it for a year. I'd be better off buying it.
At least you get to keep it with XBL it seems. Also, I pay for XBL for its service, not the free games. PS+ offers nothing besides its rental service that I'm interested in, so it holds no value for me.