I've played with and against Welbeck and obviously at that level, he was a monster purely because of his athleticism. At this level, along with Balotelli, they're both pretty ordinary.
Balotelli has Welbeck's athleticism but when he cuts in on his right, Balotelli has one of the best shots in the game. Other than that, if you consider the importance of first touch and being able to pass, that's not enough - they're both substandard. They obviously have a certain bit of talent, but the game's no longer about athlete's. You can cheat the stats for a while, but you'll eventually get found out. Balotelli's lucky he has somebody who has a lot of faith in him in Mancini. I don't think many other top level managers would want anything to do with him.
What Welbeck has over Balotelli is reliability, work-rate and probably intelligence (though by no means is he a clever player), all drummed into him from being brought up in England.
Welbeck's more of a man as well, but Balotelli is more talented.