I'm not sure
exactly what you mean by "stiffer" or "more stable." If you mean less heeling, I doubt that there is any appreciable difference between the C30 and C320. You can make any boat heel less by falling off the wind and/or reefing the sails. If you are buying new, go for a furling main system. It allows easy infinite reefing under sail from the cockpit, so you can adjust quickly to changing wind conditions and keep everyone onboard happy. Hope this helps.