New Vegas suffered from too many game mechanics and a faction system that effectively locked out quite a bit of content depending on a few choices, as well as its assumption that you'd be a good boy and did what you were told. I sequence broke like a champ, because it's a free roaming game and shut up Obsidian, and as a result some sequences clearly assumed that I had not already heard about, solved, and cemented over certain quests. In its favor, there is a lot more world building and characters are better developed.
I could come to terms with (and then ignore almost entirely) a lot of the new game mechanics because they built on Fallout 3, but I imagine that mess would be incomprehensible to someone who did not already go through Fallout 3 and know the ropes. Fallout 3 is a simpler game, but in my opinion that works in its favor. Characters aren't as developed as in New Vegas, and many are simply two-dimensional, but they still stand head-and-shoulders over the Elder Scrolls series. Sequence breaking isn't really possible, as the game responds appropriately if you, say, skip Moriarty and go straight to Rivet City.
They are both great games, and you should honestly play both at some point. I would definitely start with Fallout 3, though.
