...Glass...because I said so.
Hah. Actually, Ive only had glass tanks. Acrylic resists cracks/breaks/leaks a little more than glass does, but obviously it scratches much easier. Acrylic comes in customized shapes/sizes, but nowadays glass is beginning to as well. I have seen someone remove scratches from the inside of an acrylic reef tank without draining the water and it was returned to new. With acrylic, you dont have access to the entire top of the tank because they are not fully open. Ive heard that acrylic is more of an insulator against heat/cold.
IMO, glass is better. Just be careful with it, thats all.
***Just a side note, check out this quote I found while cruisin online: "Don't waste your time with any fish tank smaller than 55 gallons (4' long). It is difficult to achieve stable water chemistry in a smaller tank. Most of the world's interesting fish either need at least 55 gallons of space or a bunch of companions before they'll exhibit their natural behavior. It takes a certain effort every week or two to maintain a fish tank; you'll find yourself getting very annoyed if you have to drag our your maintenance gear for a tank that seems puny. Finally the high-tech filters that you will want to have aren't really cost-effective or space-effective on a tank smaller than 55 gallons."*** I dont know, my fish are pretty interesting, even though they are stuck in my "puny" tank! what do you guys/gals think.