I'm a fan of plywood, basically because I'm cheap, but also because a properly-constructed plywood tank will never have a catastrophic failure, i.e. dump hundreds of gallons of water on the floor in a short time. There are tons of threads here and elsewhere with construction tips, and anybody with ten fingers should be able to build a tank exactly the size/shape they want if they can follow instructions and work carefully. The hardest part is dressing it up and making it look pretty; I don't bother anymore, because my tanks are in my unfinished basement, but a tank can be finished to look good with any decor if you do it right.
Glass...I had one catastrophic failure with a commercially built tank, many years ago, and I haven't bothered with large glass tanks since then. I'm even less impressed today, since makers seem to be using much thinner glass than they once did. 6-foot 100 and 125 gallon tanks were made a few decades ago with no need for a center top brace because of the glass thickness, and experienced no problems. Today practically everything needs a brace to prevent the wafer-thin glass in use from bowing. Making your own is possible but takes some skill.
No experience with acrylic, not in love with the idea of easy scratching, but if I found the right deal on one I'd probably try it. They're held together with solvent cement, which should mean that they are essentially fused into a single piece...so why do we read about seam failures? I guess you get what you pay for, like almost anything else, so I would want an established maker with a good reputation if I went this route.
I agree with
Cich Mind
that a depth of at least 24 inches would be a good idea. If you go plywood, bear in mind that standard sheet sizes are 4x8 feet, so if you built the size you mentioned there would be a lot of waste. When I build a plywood tank I want the biggest I can get into the space allotted and like to use the whole sheets of wood; I usually just have a few small scraps of plywood left after a build. Refer back to "I'm cheap"...
