agree.. porous and weight should be your main concern.. although shape should be taken into consideration too as poor scaping can and will make you crazy every time you look at your tank.
also, the equipment placed in the return area, do they need a constant water level? cause return chambers usually fluctuate if you don't have a good auto-top up installed.
didn't see how tall the sump tank would be but just make sure there's enough room for equipment.
also, access to the sump tank would be from both sides? (i understand the front side has no access)