I use 1/2" MDF, mainly because I several sheets of it in my garage, a present from the previous owner of my house.
This works fine for me as it is east to cut, especially with a jig-saw, and unlike ply, is reasonably weighty which means models displayed on it are less likely to be knocked off by careless dusting!!!!
I just edge it using lengths of panel moulding available from hardware stores, cut on a mitre block.
This works out real cheap.
Karl