I am so undecided about whether to get FO3 or NV. I′ve seen gameplay of both and I enjoyed watching FO3, but I like those western-guns and... Arrghh!
FO3 is fun to explore. You enter a random subway or building and you're sure to find a surprise or a good fight.
FO:NV is fun mentally and morally provakative (makes you think and question your own morals, which is fun for some) and it's fun to replay and see how a different character plays differently.
FO3 is fun for a playthrough for sure, but you gotta realize that you can only DISCOVER that random Mexican ghoul with a party hat guarding the unique flamethrower once; afterwards he loses his "LOLWAT" factor.
FO:NV, not everyone enjoys having their morals put into jeoprody, or having to think intensively about every little thing they do.