True more total volume is usually best. But with that said you may want to save some space under the stand for new improved ideas once you get going. A 20 gives plenty of room for pump box - 2 baffles and an return area large enough for big skimmer and a couple of reactors. A breeder 40 is better of course but also a bigger footprint under the tank. Really depends on what you want in the sump.. All mechanical sumps can have smaller footprints. Larger sumps are usually dedicated to sand, rock, mud, macros, refugiums, etc...
