Glass - doesn't scratch like plastic so look nicer longer. Plus you can add lights for plants, etc. simply by hanging those on top.
I have a DIY wood canopy that matches the stand. I think that looks better than a simple glass top, but that's personal preference.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.