It probably only one way to find out is to wait until winter comes and see what make most sense and where you want the heat the most, along with their efficiency and shop's insulation and how often you have the door open.
no NG down either roads to hook up to? How long the propane tank out back will last you with the slab heat on, at 50* or 60*? Heated slab set at 60* is icing on the cake in my dad's man cave, even when it's below 0* outside, granted his man cave is heavily insulated. He told me it cost him $1/day on average; ~$30/month, but your shop is roughly 6x bigger and not as well insulated.
Crazy idea, but can make it very economic if there is something to buy (or build...?) to heat the slab with either waste oil & propane/NG, to conserve propane.
another thing to consider is to find out what work best for you on what you don't want to form condensation!!!!!!