As has been alluded to, the first most basic decisions is whether you want a lower capacity, low power
system that runs longer, or a higher capacity, high power system that runs shorter. The low capacity system can (and probably should) be DC, where the high capacity system will likely be AC.
Related to this is when you expect to make water. The best place is out in open water where there is a minimum of silt and other crud in the water, which implies that you are making water while underway. While underway, do you normally run your genset? Is this a power cat, or sailing cat? If it's power, then you might already have ample DC available while underway, and noise is unlikely to be an issue. If you have to run the genset to run the watermaker, then I agree that the genset noise will be greater than the watermaker noise.
We opted to make water while underway when water quality is best. I've subsequently learned how important this is because the filters will otherwise plug
up after just a couple of hours if you are not in very clear water. Our cruise
time is typically 3-6 hrs, so we sized to make water in that windows. Unlike a lot of power boats, we do not run a generator all the time, so we went with a DC watermaker. The final selection was a Spectra Catalina
which makes 12 GPH (about 50 lPH).
The other thing that was important to us was a system who's operation is automated. Some people prefer the simplicity of a manually operated system and don't mind attending to it. We wanted something that we could just turn on and turn off, and that could be left unused for weeks at a time without pickling. The Spectra automatically monitors pressures and salinity levels, diverts water as needed, does a freshwater flush when done, and will automatically re-flush weekly when unused. I'm sure other systems offer this too, and I think it's an important thing to have thought out as part of your selection process.