Don't forget to include the Linhof TechnoRollex back in the list of unaffordables. I'm sure it's exquisite, though, if it's anything like my Super Rollex 6x7 back.
Another interesting option is the Shen-Hao 6x17 back, which also does 6x9 and 6x12 and fits on a 4x5" camera with a Graflok or International type back. If I were looking for such a thing, I'd probably go with that one, because it is the most versatile and it got a decent review from Kerry Thalmann in _View Camera_ magazine. Wouldn't it be neat to carry a 4x5" kit and occasionally be able to go pano with a format wider than 4x5"?
If you're looking for something that's quick and easy to operate, the Horseman is probably better. From most reports that I've read, Horseman backs are more reliable in general than the Calumet backs that slip under the glass.