I prefer Fallout 3 to Fallout: NV.
The style of gameplay is more or less the same with some tweaks and a different storyline. I think I prefer Fallout 3 to Fallout: NV because in FO3, you can still play the game after the you finish the main quest so there is a "continuation" of sorts. in FO:NV once you finish the main quest, that is basically the end of the game. The DLCs for FO:NV must be played before completing the main quest. Of course there are mods that allow you to continue to play after the main quest is finished.
I've probably played FO3 either 4 or 5 different times. I've only played FO:NV once. I even continued to played FO3 after playing FO:NV. I kinda lost interest in FO:NV to the point that I did not even bother buying the last two DLCs (Old World Blues and another I can't remember).