Re: On a 27' boat, I'd go with a self-contained system.
Granted, I don't do a lot of extended cruising. But I and my wife do often sleep on our boat for a night or two, sometimes more.
I went through the same type of decision right after I bought my 1980 Hunter 36 -- either completely redo the fixed toilet with holding tank system-- or scrap it and install a high capacity portapotti with fresh water flush that also had msd fittings. I went with the latter and to date I have not msd'd it. After a two day stay on the boat, I just remove the bottom portion from the boat and dump/clean it at the marina's designated portapotti station. While a little bit gross to do, this is certainly a lot easier than trying to get my boat into the very tight msd pump-out berths we have in Sausalito. If the portapotti is used on day sails (particularly if for the #1 type), I don't bother to empty it; maybe even for a few months. I have no odor in the boat. Also it's a lot easier to instruct guests how to use a portapotti than a fixed system with the right sequence of pumps and valves operation. Also installation and cost of a portapotti is very favorable.
Point of this recount is: Do review Peggy's portapotti option.