Fallout 3 I liked, but New Vegas I find even better, even though you know the game world already.
Fallout New Vegas takes place in a less damaged part of the world as Fallout 3, the game world as a whole is much more compact. This has the consequence that it is better guided by the story and the world is just normal and at least I felt the whole thus as "real". Others have written here, there would be less to discover, that's probably true, I felt it was positive because it has not frittered away. The world is still big enough!
The character system continues to offer more customization options than other modern role-playing games, the combat system is also top.
The graphics are still good, because the world is very dreary without, the old engine is so well understood and has the advantage that it is going very well also on not current computers.
Negative I do not really noticed if you like the apocalyptic scenario can safely access.