[Glimmerman911]

You will lose more flow piping the sump return pump into the manifold compared to running it right into a bulkhead. There would be fewer bends, and less head height.

I would get the tank drilled low in the back, and use the sump return to put flow behind your rock where the manifold would have trouble reaching, that is my only complaint about the manifold system is that it is tough to get flow behind the rockwork unless you drop the outlet to the bottom of the tank behind the rocks, which puts too much PVC visible IMO.