I preferred Fallout 3. For starters, not dealing with the silly gray-scale political crap. Yaay, we've got the corrupt but less brutal group of Californians from the two games I didn't play. On the other hand we've got the chauvinist, slave trading, themed horde (who's leader clearly missed a few history lessons). Or, if you're just a greedy sumovabitch, you can just take it all for yourself. Yeah, realistic choices I'll give you that (Be greedy, be greedy and selfish, or hope the system works). Sorry, I'd rather have the choice between 'kill all' and 'give everyone a chance'
Secondly, gameplay worked out about the same way. I feel like VATS was more useful in FO3, just because of the AP costs. Melee was definitely more fun in NV, and the iron sights did help me compensate for having to actually aim. But I didn't like having to meet certain levels just to use certain weapons, that part of NV was a pain, realistic, but a pain. Hardcore mode was a nice touch too on the realism bit. Overall, NV was more realistic, but part of the fun of FO3 was that I didn't have to actually worry about that kind of stuff.
The crafting in both of them was..well in all fairness I can't be fussed with crafting, so I'll just shush.
And for the depth of the world. I could wander FO3 without ever seeing anyone but maybe a raider or a scavenger. In NV, there's a gajillion towns, most with their own faction, and you're more or less required to talk to all of them at some point. If you want to admire the ruined scenery, FO3. If you want to run from town to town, talking to people, go NV.
In the end, They've both got their share of problems. Neither has the best writing I've ever seen, and they've both got some unsettled bugs (NV also has some radioactive bugs, or more of them anyways). I think that what you really ought to do is just save your money for Mass Effect 3; then get used copy of the game of the year edition of FO3 after you beat that.