I have owned all three boats you mention H18, G-cat 5.7, and the SC20, the 5.7 is faster than the stock H18, the 5.7 is difficult to right solo without a righting bag or pole, the SC20 will horizon job either boat and can be righted solo using the Hyfield levers and shroud extenders, no pole or bag needed.
I posted a vid of my first sailing of the SC20 on beachcats, here's the link again:
http://www.youtube.com/watch?v=31Telh7OM0M
The G-cat 5.7 is a great boat, I soloed mine all the time, she's also a great cruiser with the front tramp. A Supercat 19 is worth a look of you want to keep the SC20 in sight.
--
'life is too short to drink cheap beer'
--