Haven't played either yet, though I assume it'll be an easy victory for TW3 at least in my case.
The fact that it's Bethesda developing FO4 is already a negative indicator to me. It'll end up like Skyrim, which I've played more than I'd like to admit. However, the reason I enjoy Skyrim isn't because of the game itself or its story, but because it's a big sandbox for mods and, thanks to mods, I can roleplay as awesome stuff like; a traveling Bard accompanied by his childhood friend, a witch hunter cursed by lycanthropy who travels the roads in a wagon, or a badass Shaman. Mods made Skyrim and they'll make FO4, and hopefully Bethesda won't do something stupid like kill the goose with paid mods.
New Vegas, on the other hand, drew me in because of its story and its factions. Hardcore mode was awesome and I could get into the world with relatively few mods. Pretty much I only used a house mod and some clothing.