I think Saints want Mario more than D'Brick. They just drafted Jammal Brown as #13 overall last year. Sean Payton said Brown would be moved to left tackle. You don't normally draft two top tackles at high first round in two consecutive years, you'll end up with a right tackle earning left-tackle money. Even Saints aren't comfortable on Jon Stinchcomb in right tackle, they may address this issue in later rounds instead of #2 overall.
On the other hand, although G.Grant and W.Smith are also former first rounders, they didn't play to their potentials. Besides, they were drafted several years before Brown and Saints may have done waiting for one of them, possibly Smith.
I can see Saints draft Mario and, if they trade down, it won't be to a team taking Mario.