However, no matter which one, you'll still have hoses and a holding tank, all of which use up a LOT of already limited storage space on a 25' sailboat. So there's an alternative system worth considering: an "MSD" portapotty.
The "MSD" designation in the model name/number means it has fittings for a pumpout line and vent line, and is designed to be permanently installed (actually just sturdier brackets than portables, so you could still take it off the boat if you absolutely have to), which means that although it's still called a PORTApotty, you don't have to carry anything including urine jugs off the boat to empty it.
A 5-6 gallon model is household height and holds 50-60 flushes...you'd need at least a 30 gal tank to hold that many from a manual marine toilet and I'm not sure a "composter" can hold that much poop and organic material needed to absorb the liquid in solid waste (which, btw is about 75% water). No plumbing needed except a vent line and pumpout hose--so no new holes in the boat, no electric connectons, and -0- maintenance needed except for rinsing out the tank--which you can do with a bucket while it's being pumped out. Total cost including the pumpout hose and vent line is about $200--a fraction of what you'd spend for toilet, tank and all the related plumbing needed. And the best part is, you have all the advantages of a toilet and holding tank without giving up a single square foot of storage space.
If this idea appeals to you, check out the Dometic 975MSD Sanipottie.. The sbo.com online store has it for an excellent price.