I like both in different ways, but I must say that vanilla I prefer Fallout New Vegas due to gameplay changes it has over Fallout 3.
I love the underground in Fallout 3, sewers and Metro tunnels are just great to explore, but as I already mentioned the gameplay is much more to my taste in Fallout New Vegas.
A soldier’s always frightened. It’s what you do with the fear that counts. Fear can turn you inside out, Or you can make fear work for you. Use it like another weapon. Fear’s a funny thing. It can make you weak kneed, make you pee your pants make you whimper like a baby. Or fear can make you run faster, hit harder.