New Vegas was way better.
In 3 it's a lot harder to get caps and find ammo, plus 3 felt like a huge badly done scifi clusterforget. New Vegas, however, felt nice and filled out, caps and ammo weren't so difficult to find, plus the story was so much more pleasing, since there were tons of possible endings. In addition to that, instead of feeling like a scifi orgy, it actually had a believable storyline.