Biggus, one can compare aspects of the performance of each - such as their respective ability to get players to play beyond their apparent limitations - with reasonable fairness, but not the whole of their careers.
Wenger's spending restrictions are not "totally" self-imposed. That is simply bullshit. United have had vastly more revenue than us for the majority of the time Wenger and Ferguson have managed in parallel, and Ferguson has not been under pressure to save.
If these absolutes were reasonable you could slam any manager in England for not earning his club promotion to the PL and winning the title. Including League One, League Two, the conference etc.
That said, I do think Ferguson has outperformed Wenger because I look at the wage table and see that he has won quite a few titles from second spot on the wage ladder, which is the most strongly correlated factor when it comes to league performance. This is particularly true in recent years.
And finally, I'm not really interested in discussing this objectively, I dislike, disrespect and despise Ferguson and everything he has "achieved" is as dust to me.
