This may be against popular opinion, but I actually think there is more depth to the character of Michael Scott than there is to David Brent. At least in The Office Seasons 2-4. You get a sense that Michael Scott was actually a competent salesman, but was promoted beyond his station and now has a job that really doesn't suit him. I also think there is much more sympathy for Michael, you see that all he wants deep down is a family that he can love and that loves him back - He tries desperately to create this atmosphere in the workplace because he fails to achieve it at home.
That's not to take anything away from David Brent, who is also a great character and well-realised. I just think there is more depth and more layers to Michael Scott.