IMHO, I think it depends on what you want to stock it with. If you like soft corals a nano cube with a few mods would be nice. OTOH, if you want to keep a lot of SPS, I think you'd be better off with a sump and a MH pendant. If you have a sump and a open top your options for heat mgmt. and skimming are wide open.
You wouldn't want to put up a $1000-2000 20gal(just tank,stand, and equipment) to keep leathers, zoos, and mushrooms. But, then again you might...reefers are strange like that,lol.
~Dirt