I'd give it to Heat. They won game 6, one that seemed to be for Spur's taking. They've got the psychological edge now. Chances are, Spurs are mulling over that near-win. And this can play a HUGE role in deciding Game 7.
Last but not the least, the crowd would favor Heat. There were 17 instances when an NBA championship reached game 7...and in 14 of those instances, the home team won.