Even though i prefer my XBOX over PS3 i'd go for PS3 seeing as its for a 5yr old. I've got both and my son who's 3 easily plays the PS3 but struggles with games on XBOX because the controller is slightly heavier and bigger. Not sure if this would be a problem for a 5yr old. My son also prefers PSMove over Kinect with games like start the party being fun for the family.
PS3 also has a fair few exclusives kids love like little big planet, ratchet and clank, sly collection being a few.
But then the XBOX has Indie games which are great fun for 80ms points and theres probably tons of games from that selection he'd enjoy which could keep him happy if you bought 1 a week or something to add to his proper games.
Also as someone else has said, XBOX has really upped their game with the latest update. Youtube, Dailymotion (videos, music etc), Muzu (music videos), Lovefilm, MSN Videos (news, classic tv series) and loads more apps making the machine even better but whether any of this stuff would even appeal to a 5yr old.