Well here's an update. I decided to go with a slab. For some unknown reason I thought it would be a good idea to dig down slightly and level the area, which turned out to be a minor mistake. It costed me double the amount of base rock needed, about 70 yards, and two days time spent on a jumping jack and plate vibrator. I will say I built it back up and compacted the heck out of it. It's formed up now with an 8" wide stem wall on 3 sides. The slab will be 6" thick with 1/2" rebar on 18" centers. It'll take about 40 yards of concrete.