A Utd fan and a Chelsea fan debating the Drogba vs Rooney issue is probably as pointless as a Liverpool fan and a CHelsea fan debating the Lampard vs Gerrard issue as both minds are already set and there is never going to be any convincing.
Bottom line is Drogba and Rooney both has stunning seasons last season. Drogba may have been hindered by his hernia problem and going away for the ACN and Rooney may have been hindered by the fact the team around him was not as good as the team around Drogba.
They are different players with strengths and weaknesses the other doesn't have. They are both excellent goalscorers so there is little point comparing that part of their game. Rooney is better than Drogba when playing from deep, bringing other players into the game and attacking on the break. Drogba is better than Rooney at bullying defences, at set pieces, and at defending set pieces.
When push comes to shove individuals don't matter - what wins the league is the team rather than an individual. Rooney and maybe Vidic are the only two United players who would get into our first team reasonably comfortably. When you flip that question we have far more players who would improve the United side.