If you haven't had experience with a porta-potty, it may seem gross at first blush. We've had them on other boats, and they are easy. You put some of the RV black tank stuff in them, and there really is very little smell or hassle.
Having said that, we like the fixed head on our 25... but that boat has the room for it. We have the RV type head that uses a fresh water flush, and I greatly prefer that over the typical marine head. Truly, it is like a big porta-potty, with the exception that you can't carry it around. The nasty smells one often finds in marine heads usually comes from the sea water used for the flush... and the critters in that that die and stink in the hoses. The other advantage of the fixed head in the 25 is the 9 gallon holding capacity - you can go a lot longer with that compared to a 2.5 or 5 gallon porta-potty.
Plumbing a fixed head in a 22 means that something has to go... there is only so much room, and the 22 is efficiently designed to utilize every inch. I recall someone here who eliminated the dinette to build in a enclosed head in their 22. Frankly, we use the dinette all the time... and sleep in the v-berth... and use all the storage cabinets... and we like our fixed head in the 25. But, there is nothing in the 22 that I would give up to install a fixed head. From my perspective, the porta-potty is the best solution in a 22; the 25 (and its extra space) gives you more options.