Mass Effect 2 was an extremely fun game but after you play through it once maybe twice things are pretty much the same. Sure you can make some different decisions as the game goes along but all in all your going to be repeating the same missions.
Fallout and Fallout NV are more open to exploration. You could max yourself out in level without following any of the storyline quests and still have places to visit and expore. If I was you and had to choose between one or the other I would go with the Fallout Game.