Balotelli can be great, he was VERY good for Italy, without question while Welbeck is a pile of poo. The difference is Balotelli has had multiple games where he's been brilliant, as well as multiple games where he's been useless, Welbeck has had a few games where he's been pretty good, and loads where he's useless.
ANyone that can watch both players and think they are of similar levels of ability is mental, however, Balotelli has all the other crap around him to think about, he's going to miss 3/4 of a season banned every year, he's going to be in jail for like 1/3 of his life most likely, he'll get at least 4 year long injuries after stupid house fires he starts himself for a joke, and he's going to be in court being sued for child support for another 1/3 of his life.
He's really not worth it, last year despite all that crap he got almost 50% more goals in the league, with circa 10% more shots, and he started 14 games compared to Welbeck's 23, he was used less, more inconsistently and frequently out of position(same is true for Welbeck to a lesser degree). If Balotelli was used more frequently this season, got a chance to get some form, and the team was better suited to him he'd have done better. Balotelli as a lone striker with two slow wingers who are a bit lazy and two midfielders who don't get forward much isn't the way to get loads out of him, he's not got Walcott pace to run in behind easily and he hasn't got Dzeko strength to win crap on his own. A lot of games he plays he's left pretty isolated due to the team setup.
He's better than he's showing, he's better than Welbeck, but he's not worth the aggro and he doesn't really suit the team City have, but if he stayed and started every game, he'd still do pretty damn well.