Stiffer in what way?

Are you talking the ability to resist twist? The ability to resist bending from fore to aft or port to starboard? The inherent flex of the ply itself, as in panel deflection?

A great deal of the torsional (twist) resistance can be achieved through the layout of the hull chines themselves. The same can be said for both of the others (side and length) If you want additional strength in these two areas you can add a ribbed stringer made from WRC or foam fore to aft under the deck, another on the inside of the hull and a couple of nicely shaped, ring bulkheads just in front and just behind the cockpit.

Panel deflection can be handled by the layup schedule. For a boat the size you're talking about there will be hardly any weight savings at all from carbon. If you're feeling really flush right now, go ahead and do it in carbon. I'd build it in S-glass and put the savings into a case of beer or gas money for a road trip.

Chris