One important difference between the two that isn't often brought up is that FO3's setting is post-apocalyptic, but FNV's setting is more like post-post-apocalyptic. So if you like your Fallout to be properly post-apocalyptic the Capital Wasteland is where it's at.
The problem is the great war was 200 years ago at the time of Fallout 3, so it should have been "post-post apocalyptic." So you're statement about Fallout's setting being "properly post-apocalyptic" is dead wrong. It has been 200 years, people should have been rebuilding, farming and have a working economic system. There should have been way less radiation and living trees and plants everywhere other then Oasis. There should have been farms/crops and so on.
Fallout takes place 84 years after the great war and they have, farms, working economic system, trees and people building new towns out of stuff other then wasteland junk. Fallout 3 again 200 years after the great war and everyone was sitting around with their thumbs up their butt, living in a radioactive crap hole.
The biggest problem with the setting of Fallout 3 is that it takes place around the city of DC. DC is the capital of the United States, and therefore would have been the most important target during the great war. It should have been nothing but a radioactive crater and yet it's in better shape then Las Vegas which wasn't even it that bad. Every land mark but the white house still stands and is in damn good shape. Every city in the west has been nuked off the map, any building still standing has been gutted by fire and looted, long ago.
If you want a "properly post-apocalyptic" setting go and play Fallout and see what a true "properly post-apocalyptic" Fallout is, and then play Fallout 2 and Tactics if you haven't already.