Oh look another versus thread that will just end in flaming, bashing, hate and childish "nuh-uh, my game better than yours!" debates.
http://www.gamesas.com/topic/1141845-fallout-3-and-new-vegas/
http://www.gamesas.com/topic/1183655-new-vegas-vs-fallout-3-part-2/
http://www.gamesas.com/topic/1163554-fallout-3-vs-fallout-new-vegas/
http://www.gamesas.com/topic/1343138-fo3-vs-fonv/page__p__20249981
http://www.gamesas.com/topic/1249906-new-vegas-or-fallout-3/
And there's plenty more similar threads out there, you can find your answer in any one of those.
Honestly though, get Fallout 3 first, play through it then get New Vegas later on. New Vegas is the superior of the two but better to play F3 first then NV so you can see how many miles ahead New Vegas really is.
On a separate note, these type of threads should be considered flame-baiting, as more often than not they tend to get locked due to the reasons stated above and thus should be closed on sight