F3 by a long shot. while New Vegas offered a mildly longer story, F3's world seemed to have more substance and its story was more influential.
I came across 10x the bugs in New Vegas, and I played F3 for nearly 3 times as long.
I kept coming across dead end conversation options in NV's branched missions, making them feel more linear.
Voice acting was better in F3, but more voice actors in NV.
F3 has quicker loading screens, and less of them even though both games were roughly the same size.
F3 never crashed on me, FNV did - several times.
Music, NV.
Graphics - identical other than facials, where NV wins again (just).
Companions? NV
Perks and levelling? F3
Exploration and misc. stuff with a more believable feel? F3. Exploration and misc. stuff that's just crazily unlikely with a less-believable feel? NV. So basically get F3 if you want a more believable post-apocalyptic game, and NV if you want a significantly less believable post-apoc. game (on this front).
F3 was just overall more immersive and engaging. I like to side with "graphics are not too important" but with NV I could see clearly the markings on the side of mountains distinguishing floor mesh edges because of dramatic texture changes, and floating details on surfaces that should've been on them; and invisible wall and floors were EVERYWHERE. I rarely saw any of that in F3.
Overall, get both - they're pretty piss-cheap these days but get the DLC packs with then. But if you HAVE to chose between them, F3 is a much cleaner, open, influential, immersive, enjoyable experience.
(Xbox 360)