Danno,
I've done hull repairs for over 20 years on my cats. I've done them both ways with fiberglass and also with marine tex. It is my opionion that all the fuss of doing it with fiberglass is not work the effort. Marine Tex has worked fine on my Supercat , Inter and Hobie catamarans. If you are going to do it don't worry about sanding the bottom perfectly smooth. Let the sand on the beach do it. Also don't mix it in a big glob because it will heat up fast and cure before you know it. After a year or so just do some spot repairs with marine tex if you need it. Just depends on how much you drag you boat on the beach.

Good luck,
I hope it works out which ever way you choose.

Mike