Frozen! With an extra bit of the topping if they'll do it. Also, fyi, the lines in Hog's Head are always shorter and entirely inside if it's a warm day.
We were there yesterday for the first time and we got two of each (between four of us). I think I liked the cold slightly better, but both were great. Order both, try one in the morning and one in the evening!
I used to be addicted to the frozen, but my last trip I ened up getting equally addicted to the cold. So I guess I'm an equal opportunity butterbeer consumer.