Fallout New Vegas is better because it was written by Chris Avellone, the same guy who wrote Fallout 1 and 2, and was developed by Obsidian Entertainment, most of the surviving team from the original development studio behind the Fallout series until Bethesda bought and forgeted it (I mean in terms of the story and atmosphere and some gameplay elements; most gameplay was definitely improved).
Chris Avellone is a loving legendary writer, and I was more entertained by his story than whatever that was in Fallout 3. The only reason the game "sucks" according to people was for the same reason they got screwed out of turning KotORII into the best Star Wars game ever; their publisher (Bethesda for Fallout, LucasArts for KotORII) forgetED them over.
Bethesda had 6 years to make Fallout 3, but they could barely dime out 11 or so months for Fallout: New Vegas. Similarly, BioWare got something like 3 years for KotOR1, and yet Obsidian got 11 months for KotORII.
loving handicapped publishing decisions are the reason why the game is buggy. Had they been given the problem development cycle, New Vegas would have been the highest rated Fallout in the series.
Just my 2c.