Fallout 3 and NV always just felt like a lesser port of Oblivion to me, just the same but worse in pretty much every way. Something that bothered me about Oblivion was that I could be stood there, repeatedly slashing a guy and it would take about 100 hits to kill him, but when that becomes 100 shots to the face, it just goes beyond ridiculous.
Plus Fallout pretty much just misses out on magic, one of the best things to mess around with on Oblivion, which also leads to an (almost) complete lack of original weapons in the wasteland. Add the fact that the main form of combat in Fallout 3 and NV is ranged, something that the Gamebryo engine has never done well, and the only reason left to choose Fallout is the world if you prefer it. I do not.
If the older games are taken into account then that makes the decision even easier for me, because Daggerfall and Morrowind just blew Fallout 1 and 2 out of the water as far as I am concerned.