To me it seems as skyrim is more extensive to me (more side quest and goodies) as just the game itself and no dlc or anything i usually just play the main story line and dont bother with any else unless its DLC.
Though i have to say Fallout 3 has an amazing story and intresting to play through and thus how they extended its ass kicking with an dlc that extends the story further.
FalloutNV however feels alil cut shorter so it seems more fitting to the vet of fallout games while the new guy happens to be dumb struck with all the new features of the game.
Skyrim i felt as i said more extensive on its side quest. but still offers a good story the least. Plus the new DLC coming which will involve vampires which ill pick up skyrim again for that.
In my hand i'd vote for the fallout series sorry but fallout 3 and NV have currently more bright sides in my eyes.