I'm going to update this thread. It has been almost 4 years since I asked the question and since we are now at sea and using our watermaker I can tell you what we ended up with and why
We bought a spectra ventura 200T (tropical use). I installed this myself and beyond needing to be as supple as an olympic contortionist to fit everything is small spaces (not spectras fault), the installation was relatively easy.
We chose the spectra because 30 liters per hour seemed about right (we use 35-45 liters per day for everything including showering saltwater off us on the bathing platform so we run the unit every third day when we switch tanks
(we have 2 x 150 liter tanks).
We run everything off solar and do not use the engine
so low energy usage was a critical factor. 8 amps for 30 liters is good so we use about 30 amps every 3rd day.
We really don't think about using it, turn it on, measure the impurities content, taste the water, switch to filling tanks. 3-4 hours later, flush and forget it.
So far - 3 months of usage, we've had no difficulties and we've not run out of water or even come close.
I expect we will use more water when we get to the tropics but that means we'll run the unit every other day instead of every 3 rd day
Looking back over this thread, i can only agree that you should be very sure of the amount of water you use and then purchase
capacity based on that usage - and best to add an extra margin.
For those that think our usage is very low - we're europeans and use much less water than most americans, also we are water conscious although we do nothing drastic about limiting it.
we do have electric
pumped water throughout the boat so it is not because we use foot pumps or the like.
We do most of our own cooking
so it is also not because we eat out all the time.