Love them both.
FO3 has on its main quest more of a tug at the heart story; and basically modernized Fallout franchise in numerous ways to work as a modern-day type of open-world RPG (and it felt extremely fresh and new). FO3 was also a lot more serious in tone, when compared to FO1+2. FO3 has more of a destroyed look to it, as well - especially when compared w/ the other FO games. Also, FO3's Main Story also deals w/ "water supply problems" and "finding the GECK" seemed like it was also aiming to pay homage to the great FO1+2. FO3 is the first major FO title to put the game on the East Coast, BTW. Since FO3 is a BethSoft game - its writing and companions are often nowhere as great as what FO:NV did w/ it.
FO:NV took the modernized gameplay of FO3; brought the ultra-strong and technically strong narrative, dialogue, & characters that Obsidian and Chris Avellone are often known to create; and the witty humor and quirkiness that both FO1+2 had. While FO:NV was destroyed and in ruins, it's nowhere to the extent of destruction that FO3 had going on. Plus, FO:NV has a lot more color in its palette, when compared to FO3. FO:NV's main story is not really as heart-tugging as FO3's "Search For Dad", but it's definitely technically much stronger and much better put together. Also - NV takes place on the West Coast, just like FO1+2.
Also, FO:NV has Old World Blues - probably the best DLC that I've ever played.
I can't see why anybody would avoid either FO3 or FO:NV. Both are excellent, in their own ways. I've spent over 100 hours w/ each FO3 GOTY + FO:NV Ultimate. You absolutely can't go wrong with either title. Play 'em both - but, since they are HUGE time-sinks, you'd probably do yourself a disservice if you played them back-to-back - b/c they're both very similar, in many ways. Spread 'em out a bit, I say.
Post edited July 22, 2013 by MysterD