FOWLR is fish only with live rock, using the live rock to process most of your waste. With fish only, bio-load is high and water changes and skimmers are the way to go. But, your water doesn't have to be as clean when compared to a reef setup, where you want to minimize bioload with low fish stocking, copious amounts of live rock, a big ol skimmer and coral. Lighting demands/costs are are generally higher in reef setups.