Here are few links for you:http://www.doylesails.com/design/fiber.htmlhttp://en.wikipedia.org/wiki/Sailcloth
Form my research, you can't go wrong with Dacron. For H18 I would go with 6 oz. for the main and perhaps 5 oz. for the jib. My personal preference is Pentex. Both Pentex and Dacron are different types of polyester. I have one of those Pentex squaretops in order from http://www.whirlwindsails.com/
If you consider all new cat designs, square-top, high aspect main is pretty much a standard. There is a bit of a dispute if H18/H17/H16 were designed for a fathead. I've read some posts from Hobie Co. people against squareheads for older Hobies. Then, most people who upgraded to fatheads are happy with upgrade in performance.
If you decide to shop for an after market sail, look also at the size difference. Some sails maker offer a bit smaller or greater sail area. If you decide to go with Dacron, always check the weight. Typically, Dacron weights are 4.0 to 6 oz. If you are a recreational sailor and sail mostly in light to medium winds, you will save quite a bit of money going with 4.0 oz. I personally would not recommend it, but go with stronger, 6oz.
As to racing, then you have no choice but doing with OEM. I would look at the price difference and then decide. Going with Hobie made sail has a number of advantages, such as class legal, solid, proven design. You will also not get stiffed on a deal. I've heard about a smaller sail-maker going out of business recently.
Have fun shopping,