I sailed an O'Day 25 for many years. The boat was equipped with a porta-potty that had a 4 gal +/- holding tank. I obtained two spare tanks for a total of about 12 gallons of capacity.
Pluses: The system was very simple, Only one moving part. No odor problems (used the blue liquid / formaldehyde based). Cheap.
Minuses: Lugging tanks of waste to shore in the dinghy was a rather nasty chore. I also found it difficult, if not impossible, to get pump-out boats to help me empty the tanks while anchored out (New England areas). Capacity was barely adequate for two adults for a long weekend.
My .02 worth of opinion: if you're going to be aboard for more than three days at a time, consider a "real" head, holding tank, and a deck-fill that a pump-out boat can hook to.
Hope this helps,