Thanks Jerome! That's really helpful. I was concerned about the strength and durability of the comptip but (hopefully) it seems as though my concerns are unfounded... This is the Square top I am looking at: http://www.forward-sailing.com/mainsails/hobie-17/mainsail-hobie-17-18.
The square top should if anything ease the pressure on the mast top as the sail will tend to fold away and not power up like pin tops, at least that's what I understand (and it made my Hurricane more easily handled in gusts when I swapped the original for the new sail). Thanks again :-)