Having played NV enough to be a little bit sick of it, I've since started a new game on FO3. The thing that strikes me most is how much more impressive it is to look at are; the wonderful urban vistas you get around DC, the palette of bleached yellows and grainy blues, the sheer scale of the skeletal architecture of the city. It's odd that the older game should have the prettier backdrop, but it does.
I played for three hours yesterday without so much as advancing a single quest, just exploring and fighting nasties and doing my own thing. Stumbling across Raiders battling Muties or Talon Mercs. Finding Paladin Hoss and his comrades. The world seemed pleasingly dynamic. I even found a Chinese pistol that sets its target alight, which I'd never come across before in all my previous playthroughs. And it was a joy not to feel like somebody's errand boy the whole time.
Though I miss the iron sights and the new perks, and though the world can feel a little lonely compared to that of NV, and the proliferation of skill books seems a little jarring (I found more in Mama Dolce's alone than in practically a whole playthrough of NV), returning to FO3 has only reaffirmed in my mind what a special game it is.
This is pretty much how I feel as well. Very eloquently put, I think. I think F3 has the edge as well by virtue of it coming first. The excitement of discovering grand new locations seems to be lost a little bit in FNV. Most locations are merely map markers, and very little else. Whereas discovering Rivet City, the Museum of History, the Citadel, Tenpenny Tower...all amazing. As good as FNV is, its difficult to recapture all that. So they went for a bright buzzing New Vegas Strip, and it does look fantastic, I admit.
The one significant upgrade that FNV has is its quest design. It is far more advanced than that of F3. In F3 you simply had a 'good' or 'evil' way of resolving a side quest, while the main quest was quite linear. However, in FNV you can resolve things pretty much however you like - side with anyone you like, or noone at all. Its very cleverly done, which is why the game's emphasis was on this, rather than exploration for its own sake.
In summary - if Fallout 4 combines the quest design of FNV and the atmosphere, combat and exploration of F3, then itll be the best game ever!