They're both very good. The first time I ever tried the Dunkel I was in grad school and had a Polish friend...he wanted me to go to a Polish restaurant that had opened with him, so I did. When we got there it was closed due to a ruptured pipe and the owner was in tears. I guess he was a little dramatic by nature and thought he was ruined because his restaurant was essentially flooded. My buddy spoke to him a bit in Polish and we wound up drinking all night with him (Weihenstephan Dunkels, on the house), and a week later he was opened up for business again.
Are there better Hefeweizens than Weihenstephan? Well, probably. I guess it really depends on your taste. But it is essentially the gold standard of the style to me, and a fantastic place to start.