Something which struck me about the writing for dialogue in Fallout 3 including character names like 'Amata', you can feel the influence of Oblivion even though it's a different universe. The vendors especially have this theatrical say of speaking e.g.
Fallout 3 Vendors: "Welcome one and all to my shoppe of wears and trinkets for the every-wastelander, browse to your hearts content!"
Fallout NV Vendors: "YEH?! What you want? You got caps, right?"
Back on topic though, I probably wouldn't play FNV as much if it didnt have mods. I'd likely choose Skyrim just on content alone and the variety of characters I can create.
Base game FNV isn't a challenge for me, i hate the DLC packs with free early loot.
Base game FO3 is better though towards late game every playthrough ends up feeling the same, megaton house stacked with weapons.