Monroe is a solid starting big in this league at either position. I like Davis and his ability to play defense, especially help/pick and roll defense, rebound and run the floor, but he's probably a complimentary player a lot more so than Monroe.
First of all, I didn't get to see as much of Davis as I did Monroe. With that said, while I think they will both end up being good players I see Monroe being the better of the two. He just seems slightly more natural and smarter. Athletic guys diminish over time. Smart guys have longer careers.