I've had and used a couple of UDS's, but now use a wsm. With a dry pan in my wsm, there's not as much moisture as in the UDS, and probably less than the ceramic cooker as well. Put water in the wsm pan though and it'll definately produce a steamier environment.
The UDS will add "fat in the fire" flavor if that's your thing. Personally though, it can be too much, especially if cooking more than just a couple of butts. It is a very good cooker for getting stuff done faster, though. However, I prefer my wsm, and I wouldn't get a ceramic cooker for smoking with since they just don't have the capacity I'd want for the $$ spent. Although I love my 18.5" wsm, I'd like the 22.5" wsm for the occasional BIG cook, and cooking ribs. The 18.5" wsm will overcook the ends of full racks unless rolling and skewering, but it's ok to just smoke half racks of ribs. With the big WSM though, you have the ease of glazing three whole slabs layed flat per rack.