I see that you also asked about getting a square head or a pin head. Hands down, the square head has more performance potential than the pin head. However, the tradeoff is that it might not be class legal for your Isotope or that you will, at least, take a portsmouth 'hit' (modification factor) for having a square head - if the OEM sail for your Isotope was a pin head. The square head acts like a taller sail in light air but when the wind pipes up, the square part will blow off and loose power at the top...exactly where you need it to. But if you do have a square top you will loose rating points, or potentially be dis-allowed in class racing.

You probably should check your class rules if you think you may want to race along side other Isotopes. Usually class rules specify that sails should be of a certain cut, material, and from a specific manufacurer. Anything else and you will not qualify for class racing. The other option is to plan for the boat to race in 'open class' under Portsmouth handicap ratings and the choice between square top, pin head, or material will not be quite as critical...just some thoughts.