The bad smell is from stuff in the seawater intake line that rots, or grows, or whatever you want to call it- it's anaerobes, actually- and when that water passes into the bowl, you smell it.
There used to be a device on the market called CycleFlush, which was a little pump that had a solid state timer on it that would flush the seawater out of the line every few hours. It apparently cured the problem, but I haven't seen it in catalogs recently and don't know if it is still around.
Holding tanks will smell a lot less if air is bubbled through them, killing the anaerobes. Some treatment systems are able to do this.
This fall, my PAR Jabsco head all of a sudden quit working nearly on the eve of the trip to take Blue Note to Virginia. I had always wanted a better head for the boat (a new one on my shoulders would suit admirably, too, but they cost too much and used ones aren't available), but as it turned out I didn't have time to change everything around, so I bought another PAR Jabsco. The old one had lasted fifteen years. It works fine. If they are maintained, they work great, they are just noisy.
Tecmas are supposed to be the best new ones, but I can't find anyone who has bought or used one, and I didn't want to be the first penguin off the ice. Frankly, I would get a Raritan, or a PAR, or a Groco. They all seem to work quite well. The VacuFlush system is very nice, but costs a great deal more and they have to install it.