Gonna necro this because I intended to reply the second it was posted but never did.
I bet it has already been established that critics and roleplay connoisseurs absolutely love New Vegas over Fallout 3, because it has better writing, more talking and all that jazz, but I personally think both games are great. I usually don’t care much for dialogue, random books with lore or how many replies I can choose from in any conversation. What I’m interested in is how interesting the world is and if I’ll find something if I decide to walk into any cardinal direction and I think both games provide just that. The National Guard Depot in Fallout 3 was amazing to explore and hunting for the different power armors and other exotic weapons in New Vegas was probably my favorite thing to do even over the storyline. The DLC for Fallout 3 were incredible, too!
Either game will give loads of hours of quality time. Go for either one and you’ll have a blast.