Michael Scott is a different character to David Brent.
Michael Scott is almost comparable to a child that never grew up and turns out to be a very lonely man, motivated out of desperation to be liked but is socially inept.
David Brent is a slimey, sleezy character who is simply concerned with himself, how he appears to others and is motivated by greed and self promotion.
There's quite a difference between the two. But I like both in their own way, Series 1 of the US office is poor in comparison to the rest of the season - the writers and to some extent Steve Carrel, had their hands tied with trying to mimic the uk series, but in series 2 they started to make their own way.
Look at Michael Scotts hair in season 1 - it's like David Brents, slicked back. In season 2 (and 3 and 4) it's parted to the side, and kind of signals that change from trying to imitate to trying create something fresh.