All depends on the game. Not all games and applications make use of multi-core CPUs, so upgrading to a quad can sometimes hurt performance if it clocks to a lower speed when running single-threaded applications or games.
Can't help you with the failure rates of PS3's vs 360's. I can tell you that the PS3 in this house died, but I think that was caused by user-error. (translation: 8-yr old child repeatedly powering it down improperly by flipping the main power switch on the back rather than shutting it down...) We've had the replacement for 9 months now and no problems because I told him if he ever did that again, I would take it away forever. LOL