Actually, I'm currently replaying both Fallout 3 and New Vegas at the moment (I kind of trade off every couple of days - when I start getting a bit tired of one, I switch the other, and so on...)
Tonally, I'm much preferring Fallout 3's Wasteland, actually. From an art direction standpoint, it just feels a lot more... cohesive. You can tell that their primary objective with the game was to make vistas that were visually striking, and then tried to fit the logic to work with that. But really, even when I first played Fallout 3 (and coming to that came as a long-time fan of the original series,) I had some gripes about a lot of things, but I really can't say enough good stuff about their art direction in that game. For better or worse, accurate or not - Fallout 3 still looks more like a "Fallout" game than anything else I've ever played.
Fallout: New Vegas... art direction is just not Obsidian's strong suit. They don't have as much experience creating a gameworld of this kind. I'm not saying they're bad artists by any stretch, it's simply that (again, simply in terms of visual appeal) that creating a visually arresting world that's a joy to explore is
kind of Bethesda's greatest strength.
Structurally, however - I liked the attention to detail that Obsidian gave the Mohave Wasteland. I like how there's a logic to how everything is laid out. You get a sense of how each settlement has managed to survive. You can tell that they really wanted to make sure that there were viable reasons that people would be living in each place - what they do, why they're there, and how it all connects to the larger picture.
So for me, it's a mixed bag. If I could get a game that looks as fabulous as Fallout 3 did, with as much attention to creating a believable infrastructure as New Vegas did - then I'd be a happy camper.