The only pro on the glass bottom that I know of is ease of cleaning, since you can vacuum/suction out gunk.
However, sand on the bottom provides a valuable part of the biological filter. I'm sure you'll hear more pros for sand than for glass, especially regarding the quality of the tank water/health of its inhabitants.
Glass cannot remove Nitrates and most reef tanks need nitrate removal for the health of marine livestock and algae control.
Glass cannot aid in pH buffering.
BurnNSpy
Well I have never had a glass bottom but I do have a 5-6in sandbed and just for looks if nothing else I would go with sand...I have had more compliments on the sand than anything else...
my vote is sand!!!!