I think it depends a lot on what you are putting in your sump for filtration purposes. Biological media tends to need a little bit longer to work so not as quick of flow through rate is as efficient. Also skimmer flow rate could play a factor also. Really it depends on what you want to do. What I would do would be get the 5.0 and make a T in the return and on the side coming off put a valve and piping back down to the return pump chamber. This is beneficial because you can "bleed" off pressure and flow without putting back pressure on the pump which will work it harder, causing more heat and shorter lifespan. So I would go with the larger pump and start scaled back and if more flow is needed then scale it up.