The geography, the details, everything about the Mojave seems like it's had x10 time spent on the landscape.
Of course, they only had like a month for each DLC.
I still like Honest Hearts the most. The landscape is superb, imo and always after visiting Zion, I find the Mojave to be extremely boring to look at. I even thought about creating a new landscape from the Zion set, because I want to have MOAR of it. Painted a sketch and all that stuff, but somehow never started working on it... Oh well.