There are really no structural design considerations to steel reinforcing in a slab on grade. It's only function is to restrain cracks after they occur and both rebar and welded wire fabric (mesh) do this equally as well as long as it's positioned properly. Because rebar is more rigid than mesh, it's easier to support but it takes longer to lay it down.
If you use rebar, I'd suggest #3's at 12-18" centers supported every 3'. For #4's, you could go 18-24" oc supported every 4'. Don't mess around with anything bigger. One downside to rebar is it's only readily available in 20' lengths, which wastes a lot in splices.
If you go with mesh, buy sheets rather than rolls so to will lay flat. 6x6 2.9x2.9 wire is fairly rigid and can span 3' or so between supports.
I'd figure the cost of the various material combinations and go with whatever is cheaper. Check out my Guide to Floor Slabs in the link below for some additional thoughts and good luck with your slab.
Want more information on What Wire Mesh to Use in Concrete? Click the link below to contact us.