Personally, I think both the DC and NV worlds are plausable for the timeline. The DC area was devastated by many nukes, had a larger population which had been used to an urban, inter-connected, lifestyle. There probably weren't a lot of folks living in that area that were used to huntin' up their own vittles - for at least 150 years before the bombs dropped. I think that in both in the amount of damage taken to that region, and the sociological/psychological impact that that kind of massive cataclysm would have on the population, it's entirely reasonable that things would take much longer to rebuild. The area surrounding Vegas is much more rural, has a much smaller population which is probably more self-reliant to begin with. Couple that with the fact that nukes didn't fall there, and it makes sense that they should be a lot further along in their development.
Of course you realize that it's hard to keep a straight face talking realism when you're dealing with giant fire-breathing ants and green (or blue) mutants with miniguns.
As for what atmosphere I LIKE better, well, I've always been more of a fan of horror movies than westerns, so I prefer the Fallout 3 world. I like the creepy feel of that game better, personally. I did play Fallout 1 back in the day, but frankly have forgotten most of it - I do remember that it was a great game, and I still have it on my shelf. I doubt that my Vista rig can handle it now.