I replaced our marine head with a C-head composter this year. Unless the boat grows to a larger boat with much larger tank capacity (mine had a 10 gal tank), I'll never go back.
My 2 cents...the thing just works as designed. The C-head is a simple and practical design. The compost tank is round and there are no hidden corners and crevices for any gross buildup. These things are desiccators, not composters, per se, so drier in the compost area is better (don't picture a wet mulch pile, cause it's not that). I did install a passive vent on mine, but honestly it's probably not even necessary on the C-head. There is no smell. We are using coir, as a media.
Down side, if there is one, is that urine is stored in a 1 gal container, so you either need to dump it daily when in use, or just have a couple more containers available on board for storage. No big deal. I add a splash (a tsp or so) of the Thetford biodegradable blue stuff to the urine container, and there is no urine odor, even when dumping. I dump it into any on shore toilet. If in a remote area, it will get dumped on land. The C-head doesn't have the capacity of the 2 other brands, but it comes with a vented cover for a 5 gal pail. The auxiliary storage pail then is vented in series with the head. You can likely hold about a month's worth of waste by increasing storage this way. What dumps out isn't gross at all...it converts it into dry round waste balls. You could probably use them for ammo, if you wanted (ship cannon not included). So far, I have just brought the compost home and buried it in the yard, but it could just as easily be bagged and dumped in a dumpster.
The ladies have (at least on our boat) had no issues with adjusting to using it. It takes less explanation to visitors than our marine head did, as far as usage directions. For men, they just need to know that they SIT on my boat. You don't sit, you don't pee on our boat.
So far, very happy with the change.