You have got to be joking, right? Have you ever played either? Jam is unbelievably addicting and obnoxiously fun to play with friends, sober, drunk, high, or not. That game is pretty much the only reason I ever powered up the xbox, other than movies of course.
I bought blitz, tried playing it a few times, alone and with friends... It sucks hairy monkey balls. I can't see it ever being much fun. Thats not to say there arn't good football games. But blitz just aint one of them.
You wont find a more enjoyable and funny game than nba jam. This is no contest. It's to multiplayer as MGS, or GTA is to sp.
-moto