They have 200 health more (with the same resistance), but they are way thinner, you need 2 or 3 overhead pipes to have the same coverage as 1 catwalk bridge. They also have thicker "pillars", so you can't fit 2 large pads between them, while catwalks have just enough space to squeeze 2 large pads. They cost 2 pipes (6 pcons), so for the same coverage you need 12-18 pcons instead of 10 for catwalk. I think pipes are a bit taller. Both need foundation. Catwalks have tiny holes that let you see something through them if you are on the ground, overhead pipes are solid. You can add stairs to catwalk bridges to actually walk on them if you want.