On heavier days: just me and son. Trapeze should be fun.

You are a little shy of ideal weight for the Hobie 16 (and others mentioned, I expect). Will probably take some time before you're ready for "heavy"....depending on how you define that.


will we be able to right a nacra 5.2 or a hobie 16?

You and son will be unlikely to right any of the the boats mentioned in all conditions without a righting aid (pole, bag, etc.) or assistance. The four of you....sure.


next issue: if I bring the three kids (about 240lbs + me (190lbs) would a hobie 16 hold us - what about the Gcat 5.0?

All boats mentioned will "hold" you, but none are going to perform well in the medium/light wind you propose.


which is faster - or is there a real difference between them?

You are unlikely to get any really good, unbiased answers to that here...especially mine! Check the Portsmouth handicap Tables at:
http://offshore.ussailing.org/Portsmouth_Yardstick/Tables_2010/Multihull_Classes.htm.
Lower is "faster," but it all depends on how well the boat is sailed.


Any other advice?

Try to hook up with catsailors near you to gain some knowledge and experience before you buy. Give consideration to getting a boat that is popular in your area for help getting up the learning curve.