my opinion:
i played both "f3" and new vegas(also both with "harder core" mods and whatnot...)
fallout 3 has the poorest, worst written story EVER. the game itself is waaayyyyy too easy.
theres a lot of stuff that is too clean, and does not at all feel like fallout.
never needed any caps(money), only "hard" scenes were the first mutants/ first deathclaws.
not to mention the levelscaling.....booooooooooo!
good thing is: it looks surprisingly good and has some good moments... like the beginning/prologue.
if you are a more "casual" type of player youll still love it
NV was too easy... well, at least the story was much better!!
but still less a fallout-like feeling compared to f3, but that is ok i guess, something "fresh".
you will need money, and "skillchecks" are much more common.
many sidequests, factions and REPUTATION!
New Vegas>>>"fallout" 3
oh and that fps-style suxxxxxxxx