Gameplay-wise, I'd say '09 definitely has the advantage. Although it has some kinks, it's a solid game.
'10 has the cartoon-ish graphics, point to pass offensive gameplay, unlockable "classic modes (Franchise, Create a Player, and Situation Mode), and up-to-date rosters.
If you want a more "hardcore" Madden, get '09. If you want the current rosters with decent graphics, update gameplay, get '10.
You really can't go wrong with either purchase. Have fun. :3