I'm curious to see where people fall on this one. I'm personally in the PDI / Invisiglass camp. They are both awesome mods, but the non-glare glass actually affects the way in which your eyes interact with the game. Depending on the lighting in your game room, it can make the playfield much easier to see. As cool as ColorDMD is, it's still just eye candy.
Incidentally, I just ordered my first ColorDMD today for TSPP.