Like stated before, I would use two. One would be the main return and the second, a much smaller one, would be back-up if the main ever failed. I am going to use a quiet1 3000 hooked to a UPS. This way if there is a power failure there will at least be some flow in the tank. Just try to calculate wat flow you want thru the sump and find the pumps you want to use to meet that number. I am going the main return external and the quiet one internal.
just my .02