I'm no expert in this area at all, but maybe you could provide us with a little more background in what exactly you're looking for? Dump body? Or flatbed? What's the intended use of the truck?
Haven't seen much of the Mitsubishis around here, so can't comment much, but the GMC (or Chevy) models are decently popular, usually more so with gas engines than deisel. See a lot of Ford and International; big landscape companies seem to dig the International while smaller, localized companies seem to use Ford.
I would go with the c5500, I plan on getting one in the fall (if i can afford it) when the 4x4's are coming out... Many guys use the c4500 4x2 and just put weight in the dump and they say they plow like a dream.