For a gas rig, those are the choices, that is if you're looking at used (there currently is no Workhorse gas chassis offered). Each has it's fans. We had a Ford, with the V10, and I was quite happy with it. I would not hesitate, however, to purchase a rig built on the Workhorse chassis though, assuming it was otherwise everything I wanted.
One Caveat on the Workhorse. Before buying check to see if it was subject to the brake recall and if so, has it been done.