I decided to go with a the concrete slab because the ground had a deep base of clay which prevented good drainage anyway.
We tinted the concrete a terra cotta color to help aid in heat absorbtion and retention. there was a drain in the center which tied into an existing yard drain just outside the GH.
I had overhead misters on a timer that ran 1-2 times/day for all my mounted and hanging plants but I watered all my benches by hand. I also had hot & cold running water. In that we were on a VERY deep drilled well, it was nice to have the ability to temp mix the water so the plants didn't get shocked by the 38 degree F well water. In the spring (Memorial Day in NH)the plants were all moved to an outdoor shadehouse and the concrete floor made it super easy for me to powerwash it and if I had an insect issue that year I'd give it a good soak with mildly chloronated water (physan strength), let it sit a couple days, then power wash it again. I just think the concrete made for much better and easier sanitation.