The core gameplay between the two is nearly identical. The differences come from the story, character development, equipment modding and quests. --- Duba Dubababa Ba Da! Dub? http://whatpulse.org/stats/users/338862/
Both games can be completed in just a few hours if you rush all the content and bypass all the stuff that actually made them GotY material and so popular to begin with. Neither game will be less than 50 hours if you actually take your time to explore, find Quests and side missions to do and just enjoy the game for the incredible atmosphere that they have. I put about 200 hours into a single character in FO3... I put about 80 into NV prior to the expansion packs.
Both games have their good and bad sides but both are great games regardless if you want my opinion. Start with FO3. --- R.I.P - My kitty 2010-2012 on 3/22/2012 She lost her battle with Feline Leukemia
Fallout 3 is worth only a single playthrough for the story (which isn't even that good in my opinion). New Vegas, on the other hand, easily warrants multiple playthroughs. --- "I gotta be really careful while shadowboxing though because if I do it enough eventually I'll just defeat the air and then everyone will suffocate."