To be honest, if a PS3 or 360 owner buys the other console, its unlikely they will be blown away with either. There will be unique qualities that are appreciated but, to be honest, the consoles are pretty close overall.
Well, except for the exclusives. Always cool to be able to play great new games. Have fun and make sure, if you have the time, to give some more low-profile gems a try (like the FIRST Crackdown, any Twisted Pixel game and Banjo Kazooie: Nuts & Bolts), k?