To me the choice would depend on your intended use. The 16' version is an excellent pocket cruiser for one person. Yes, it will handle 2 people both cruising and sleeping and for a night out and may be totally acceptable for your use. But the space is limited. Any gear stowed in the berth where it will stay dry needs to be moved to the pilot house or cockpit when two people are wanting to sleep in there. Plus overall space is a little tight for two people moving around onboard. For one person it works out very well and gear can remain in the berth and still have room to bed down for the night plus moving around is much easier. So in my book for two the 16' would work for a night or maybe even two but if you are thinking of longer trips with it will get cramped - after all it is only 16'!
I have no experience with the CS-20 but I am sure the added space would be much better for two on longer trips. BUT there will be a downside and that will be a little more cost to build and fuel economy. The actual building materials may run a few dollars more but I am betting compared to the overall project it won't be major factor. On the other hand the outboard will probably run a fair amount more as you will most likely be looking at twice the horsepower or more. And along with more HP comes more fuel burn. Knocking around local it wouldn't be a big deal but when making long runs I am sure it will add up. The CS-20 is a longer, wider and heavier boat. I know the "hull" weights are listed at only about 150 lbs different but my guess is the 20' will end up with a substantially higher real-world operating weight.
For example the CS-16 is listed with a hull weight of 500 lbs but this is the bare hull - no engine, battery, anchors, steering system................ you get the idea. My admittedly heavy and 18' version's normal operational weight with fuel, ice chest and long distance gear runs about 1650 lbs. That's a lot more than 500 lbs! And it will compound with the CS-20. The bigger the boat the more "stuff" you tend to put on it AND in it. And like me, my boats tend to get heavier with age! Seem to always find things I "need" onboard and they all add weight. As to actual fuel burn lightly loaded (minimal fuel, no gear, one person) my CS will manage right at 10 mpg. Heavily loaded for long trips it is usually about 8 1/2 mpg.
I think either designs will yield a great boat. You just need to list your priorities and decide which one best suits those.
And also, my CS won't fit in a standard 7' high garage door. It is just too tall on the trailer by about 6".
More info on my CS here - http://egyptian.net/~raymacke/index.html