Just for clarification, are you planning on running a sump? I'm assuming so, but I haven't seen that expressed yet.
Good sump design dictates that if your return pump is off, your sump is large enough to hold all the water your overflows will drain from the display tank. The overflows should also be capable of handling more flow than the pump is able to generate. Neglecting either of these principles is just begging for a flood. Just some FYI.
I'd run one return pump. It just simplifies the whole installation. I would also run a return loop to control flow and back pressure. If you just use a ball valve to limit the pumps flow, you'll build up back pressure and the plumbing will start making noise. Think of it like capping the garden hose with your thumb.