lexan (polycarbonate) is generally stronger, less stiff (not so much of a concern considering it's a fish tank), and much tougher (can absorb more energy/impact/etc before breaking). it also seems harder than acrylic (less likely to mar/scratch). i guess my question is, why use such a thick piece of acrylic on the front? also, i am wondering why not use the thickest piece on the bottom? the bottom piece sees all of the support and tank loads, meaning it's also put into bending, not just pressure (stiffness is more of a concern here too, but can easily be accounted for - use a thicker piece if need be and more supports in the base design).
all those property generalities were pulled from matweb.com and can vary greatly depending upon the polymer grade. also remember, i have no experience or studies in building tanks, so my question is also to learn a bit myself. it just seems that the polycarbonate would be a better choice for the entire tank build, granted 1/4" is thick enough with a safety factor to not fail under the tank loadings and accidental bumps.