Of course it all depends on your bio-load. I'm personally running the SCA-301 on my 40B/20L sump. I'm a mid to heavy feeder, both corals and 4 fish. Gotta say for the money this skimmer can not be beat. Won't hurt your wallet at all. Very quiet!
The SCA-302 would perfect if your looking for an in sump skimmer, in that price range.