If all you're concerned with is making sump dividers, I would use acrylic. The bigest drawback to acrylic is that it scratches easily, but that shouldn't matter in your sump. Acrylic is easier to work with and won't crack or break.
Jon
I siphon them out every time I do a water change, and this helps a little.
Some people have reported that mandarinfish or other dragonets will eat them. I bought a mandarin and it wasted away without eating any of the planaria.
Also, I've heard that there's a particular nudibranch which...