If you go plexi...go very thick so it doesn't warp over time. I prefer glass cut to size for tops. Make sure any hardware you use to hinge is rust proof as well.
yeah plexi is great because its much easier to cut and drill holes at home if you want fancy doors and such but if your running some high powered lights the heat might become an issue....im running LED's so wouldn't be a big issue.
Acrylic will eventually warp. Light, not light, LEDs, etc. The acrylic will absorb water and bow. I recommend getting some pieces of glass cut and sanded.