The composting head unit itself is generally bigger but if there is enough room for that it likely would work better as there are no other parts and no tanks. There are also some heads with a built in holding tank but the have so little holding capacity nothing is really gained.Have you considered a composting head? Much smaller and infinitely simpler.
See:I have an Alerion 28 which I love to day sail, but do no cruising. However, it does not have a true head, only a porta potty. Has anyone looked into installing a head in the forward part of the cabin? Don't use the v-birth except for storage.