Split drain or split return?

[scherzo]

I'm just about to start plumbing my 90 gallon over a 75 gallon sump.

My 90 has a 600 GPH drain and I'll be running a MAG 12.

I'm planning on splitting my returns and I calculated that I should be running about 500 GPH up to the tank.

Now my question.

Split drain (1 - Refugium / 2 - Skimmer Section)
Split return (1 - Refugium / 2 - return to tank)

Both will have valves to control the amount of flow.

Any drawbacks or benefits of each?

Thanks
-Jeff