I'm trying to decide the same thing on my Dump truck project(s)
I've got a 66 GM C50 Dump with a 5 yard bed, new tires and brakes, runs and drives but needs wiring etc.. and it leaks like the Exxon Valdez! The cab on this one is in really bad shape.
Then I've got a 66 GMC 4000 Dump, with a 3-4 yard asphalt dump bed, doesn't run, but - the cab is lightyears better than the GM
On my projects I will probably decide based on the amount, if any, of structural rust ~ ie. cab re-inforcement bracing and the cab mounting points.
If the structural areas of the cab are good - I believe most of the sheet metal and patches can be purchased new-reproduction
IF the stuctural areas are bad then I'm probably going to sell them off rather than do a cab swap as by myself, that's a nearly impossible task to do without a couple of gantry's or a forklift.
