I looked at everything, but since cost was a big factor, I went with a pole barn, then poured a concrete slab inside of it after the barn was built. My preference was an all-steel (pre-engineered) building, but there was too much of a cost difference in the slab, at least in our area. As far as interior spans, my barn is 40x50 with no interior posts - all clear span trusses.
The slab for the pre-engineered building supports the building, while the one for the barn is not load bearing - this is what impacted the cost. The pre-engineered building will require piers at the columns plus a perimeter beam and the slab itself. The pole barn slab is poured using the lower 2x6 of the barn framing as a form. I added a perimeter beam all around though since mine was on an 8" fill pad.
I still made the slab in mine a 6" thickness with #4 bars - plenty stout.