I don't have any tanks now but I used to have about 20 - all freshwater. Tried brackish, the pufferfish were cute little buggers with cool personalities but the water was a bitch to maintain.
I prefer gravel over sand. The sand looks prettier but clogs the filter and shows dirt *really* quickly. Although you could get some sand dwellers to clean up for you, crabs come to mind.
I've always fallen back on gravel over a bottom grate filter with plecostomus, catfish and crabs to do most of the daily maintenance for me. And african cichlids, of course - fish with real brains and personalities.
PK has posted some great pics of her saltwater tanks over on GFY, she's got a lionfish that is simply gorgeous.