I really considered New Vegas to be the better game, maybe it was all of the connections to Fallout 2? Nostalgialitious.
Fallout 3 had a better... flow to the story and world, maybe? I see its subjective appeal compared to NV, but I can't put my finger on it. Also I may have forgotten to do the Pitt DLC, haha.
But all the cool weapons, mods, ammo, custom ammo in New Vegas, it just gave you so many options. And the survival food/h2o/sleep aspects gave every little item you could pick up have some purpose. The Dead Money content was a little awful, I could rant for hours about it, but the Old World Blues content more than made up for that.
I think there was some mod to explore the Fallout 3 world with a Fallout:NV character, could be fun, just heard about it recently though.