How big is your tank and what are you going to use for circulation? Tunzes? Powerheads? Close loop? Seaswirls?
I try to use the least amount of plumbing as possible. Moreso always just ballvalves and unions where you might have to change out the equipment, especially the pump area.
If it's multiple returns, then use hard plumbing. If it's just one or maybe two returns, I like to use flexhose or flex pvc.