Its all about how they do it. I would be happy with either option, but I prefer Valenwood/Elswyr, and here's why.
Sure, Skyrim has variety, but it will end up having more of your traditional fantasy vibe, much like Oblivion did. If that's what you want, I wont object. However, I believe Valenwood and Elswyr have a much higher potential for the atmosphere to be as good, or better than Morrowind. Valenwood has moving trees, and the jungles of Elswyr could have extremely interesting and lush vegetation. Imagine the variety of plant and animal life they could implement. Imagine alchemy in the jungle. The desert could be done extremely well atmospherically as well, and think about how cool desert dungeons would be. As far as cities go in Skyrim, we're likely to see more castles surrounded by villages, and Bruma-esque architecture. In Valenwood, there would be cities in trees. Do you miss the tangled organic cities of the Telvanni? In Elswyr, desert cities would be awesome, oasis-type places that are huge hubs of trading and melting pots of diverse characters.
I just think that V/E have more potential to bring you to a world that you've never been to before, as Morrowind did. So much of the fantasy genre is filled with Nord-like culture and scenery. I'm sure Beth would do it amazingly, but I would prefer something a little further from Oblivion.