I love Fallout. I own all of them - including the PS2 spin-off that nobody except my sister and I seemed to enjoy, lol - and of all of them, I haven't played 2 or Tactics yet.
3 was my favorite of those two, and I'm really disappointed that it doesn't run so well on PC anymore.

I'm
almost tempted to buy a PS3, or send in my 360 for repairs -- again.
New Vegas makes a pretty decent substitute. Don't get me wrong, I love them both, they both have their ups and downs for me. For example, NV had better DLC, but while NV had more quests, I preferred the quests and their locations in 3.
At the end of the day, I just preferred exploring 3's wasteland to NV's desert. The towns were more spread out than NV, which was all centralized around Vegas ofc, which made it more enjoyable to explore the entire map. Not to mention you were paid very well for doing so, and their were the bobbleheads to find and vaults to explore. Which was another thing for me, the vaults in the Virginia/Maryland area had much better back stories than the ones stuffed into NV.
tl;dr. The exploration is what pulls me in in open-world/sandboxy games, and in that regard, 3 > NV.