Overall I would say the PS3 is considerably more powerful than the XBox 360, but you will see this benefit mainly in PS3 exclusive games. Current XBox 360 ports to the PS3 may run worse because the game engines often haven't been modified enough to take good use of the PS3's SPEs, these SPEs are the main reason why the PS3 is overall considerably more powerful.
Also the PS3 has the advantage of massive Blu-Ray storage, but in general this benefit will and has only really manifested itself in PS3 exclusive games, as for a XBox 360 game it's best to keep the game size down to fit on a single layer DVD as the drive slows down considerably while reading a dual layer one and game companies will do their best not to ship multiple discs, Blue Dragon being an exception.
