Compromise...
When space for a tank is tighter than you want to give up to one, a 5-6 gallon "pumpable" portapotty can be a good compromise 'cuz it offers a choice of having it pumped out or carrying it off the boat where no pumpouts are available. Self-contained, so it takes up no more room than any other portpotty...and because it uses so little flush water, hold about the same number of flushes as a marine toilet and a 10-15 gallon holding tank. Has the added advantage of requiring -0- maintenance. When it comes to boats under 25', I wouldn't go any other way. Over 30', I'd definitely recommend a marine toilet. In between, it's really a personal choice. Some people just don't feel their boat is a "real" boat unless it has a "real" marine toilet. Others just want to keep things as simple and as hassle-free as possible. But whichever you choose, you can definitely escape carrying, dumping and cleaning out a portapotty.