The only thing I would consider is the longevity of the current trucks.
If you've got a few older ones, put a cheap flatbed on.
If you've got a few newer ones, go aluminum or stainless.
I'm debating this right now with my 01' C3500HD Salt truck. The bed is rotting away and I can re-hab the truck pretty easily. But I doubt that I'll put anything more than another steel flatbed on it.