I would recommend holding off on making either of them a SW tank until you have your 20 up and running and got a little experience under your belt. Nanos are really a tank intended more for the experienced reefer than the novice taking a first stab at it. This is not to imply that it cant be done by someone new to the hobby, its just a lot more challenging.
In these small tanks there isnt a lot of room for error when it comes to water parameters. Loosing as little as a quart of water from evaporation while your at work all day can have a profound effect on the Specific Gravity of the tank. Additionally should your water parameters take a dive, in a tank this small you will have very little precious time to catch it and stop the train wreck in progress from happening and avert a "tank crash".
"Hold short of the active" for a while and pl;ay around with your 20 for a while and save the 10 and the 5 for later projects of special interest to you. There are some pretty amazing things that you can do with both of these tanks, especially coral wise! In fact it was a 2.5 gallon Pico Reef that got me to ditch most of my much larger lanks in favor of these little jewels of a reef tank. As ladyonyx mentioned these tanks would be perfect for a Pistol Shrimp/Goby pair and would make for a stunning display!!!
I have several small tanks like this going at the moment, one of which is a 10 gallon currently going thru and Upgrade to a SPS coral reef, that I used as a feature tank in a article I wrote at another forum. Its a really good although lengthy read thats well worth your time. Its in a tank diary format and was intended to show how cheap and how easy it was to "A" not only do a reef tank, but "B" do it cheaply using "old school" basic husbandry skills and didnt use any trick of the week equipment. It started as a soft coral only tank, was up graded a bit and housed some LPS corals and is currently being upgraded to take on the challange of SPS corals over the next couple of months. PM me and I will provide you a link as I dont want to post the link here out of respect for the forum rules here, nor do I want to post the thread here due to copy write issues.