I was recently in your situation, couldn't decide what to replace the badly worn piece of Plexiglas that was on my cabinet that I am MAMEing with.  Looked around and found Plexiglas available at Home Depot and Lowes, but it was all clear.  Went to a local glass shop, they had grey tinted glass.  I heard it was the best way to go.  I had brought in my Plexiglas and they measured it and quoted me $18.00 plus tax for grey tint, cut and sanded, exact size.  I figured what the heck, even if I didn't like it, $20 was not much to try it out.  The Plexiglas I had looked at was all much more.  I needed a piece that was a little less than 2' x 2'.  The Plexiglas was available in 24" x 18" but then it went up to 24" x 48".
I picked up my glass the next day and took it home and installed it.  WOW, it looked amazing.  I did not have to adjust the monitor settings at all.  I did not have to turn the brightness way up.  I just left it alone and the colors really popped, plus with a black bezel that I cut out from black construction paper, everything is hidden.  It looks like the monitor is just floating there.  
I would go for the grey tinted glass.  I think it looks amazing, is MORE scratch resistant and can be cleaned easier.  Worse case scenario, you probably won't spend a lot and can try it and see for yourself.
Edited - meant to say more scratch resistant.