Wonderful meta. With the first prank, sam doesn't know dean took a photo, and i like to imagine weeks later Dean emails that photo to everyone in Sam's address book!
Sam's prank is complex, elaborate and personal.
Dean's prank is simple, relatively spur-of-the-moment and impersonal

great summary of their differences.
With Sam here the humour arises out of sibling rivalry. Whereas I think humour is almost an instinct with Dean - it comes out all the time even when it might be inappropriate.and maybe i relate a bit to that. Like using rock muso names as aliases, or giving Sam a bikini inspector id etc.
Sam - remember - is the guy who wouldn't dress up for Halloween. I'm not saying Sam doesn't have sense of humour, just that it is differently driven to dean's.

See - you made me think! *goes to rest brain*