I spent last summer sailing a TheMightyHobie18 and purchased a N6.0 by the end of the season....the 18 was not an SX, but from what I know about the sx, I think my experience on the 18 Magnum should translate closely. I think everyone else here is giving you good advice so far. THe dif between the two boats is quite significant. If you had little or no sailing experience, I would point you to the Hobie 18 as it is a much more docile boat, however....your weight (especially with your crew) will be a significant problem. The N6.0 has a lot more bouyancy and can handle the weight. As far as performance, when I moved to the 6.0 it was like going from a chevy to a porche!! One thing to be aware of though is that the 6.o having been built as a performance boat will not take as much abuse (running it up on the beach) as the 18 (which is a tank!!). Given what you have said, I would definitely go with the Nacra and although it has a lot more controls, many of them you can just set at max de-powered (so-to-speak) while you learn the boat and then pick them up as you get comfortable. I would also recommend putting a jib furler on if it does not have one already so if you find yourself in a tough situation with heavy wind you can furl it while on the water to depower more.

Good luck, and hope to see you on the race course!!

Joe