1/4" Plex. You could even go a little thicker since the shelves will be bigger. If you are going with Plex, which i think you should make sure you get the edges of the plex polished. They will just heat up the cut edges and it will look smooth like glass. Good luck and looking forward to seeing the finished piece.
The shelves in my single tower look to be 3/8" thick. If you going to use them for a double wide I would probably go with the 1/2" or I have seen some use tempered glass. The original shelves in a double wide tend to sag after time, it's not much of an issue with the single tower.
Thanks everyone...I will definitely go with 1/4 inch plexiglass. I cut some shelves from a sheet of glass that I had in the basement, but it is too thin I believe. For now it works.....but it just makes me nervous.