I prefer Tamriel. The new Fallouts felt a lot smaller than the recent Elder Scrolls, and, honestly, though they were indeed amazing games, they both felt like generic shoot-em-ups. I've never found a place of extreme beauty that made me remember it for ever in FO3 or NV, and I think that's what Bethesda games are honestly about: Sightseeing tours. More than once I've had to just stop and look at the scenery for hours at a time in Morrowind, Oblivion and Skyrim. In FO3 and NV, I've only ever stopped for 30 or so seconds.
Between the 3 TES games I've listed you could go from a Mushroom filled-hill to a volcanic wasteland to the top of a snowy mountain. In FO3 and NV you could go from generic wastelandy bit A to generic wastelandy bit B, with a bit of desert in small, small sections where you had no reason to go in NV.
Bethesda is, honestly, not that good when it comes to anything other than scenery, and FO3 and NV didn't have much of that. That said, NV's story does make it quite a bit superior to the games Bethesda made without Obsidian. My rating would go Fallout 3< Morrowind < Oblivion < New Vegas < Skyrim. Both series are excellent, but I just prefer TES's non-brown scenery.