We have the plastic protectors on the blast cabinet at work and they do get dirty fast. I will clean it off with a damp paper towel when it gets too bad. If you don't use them, the blast media will be bouncing into the glass and etching it making it impossible to see through at some point. I haven't bought either and don't know which is cheaper to replace?  

What size/type cabinets are you guys using?
Have been thinking about getting one.
Compressor suggestions appreciated as well.

A shop i worked at has a USA mad sand blast cabinet its called scat blaster it worked really well but the down side with that cabinet was that it went through the guns way to fast. Literally we had to change are gun twice in a month. The harbor freight cabinet we also had was the first cabinet we had and it works i think honestly better down side was size of window and with the light it had inside was really dark. I think the harbor freight cabinet would be better if one was to drill out two holes in the top and add two new exterior spot light holders. Are scat blaster had two 60watt halogen bulbs really made it nice to see what you were working on. We only ever used 80 mesh silicone carbide and it seems to cut really nice in either cabinet. If you look around you can get the screen protector for the harbor freight system in rolls and save lots money fyi