I'm glad that you started this thread....I'm a little tired of questions like "which is better to repair rust holes, long strand fiberglass or short strand fiberglass". The answer for anyone reading this thread is NONE OF THE ABOVE. I used to try and answer these threads and persuade the OP to not use either and repair it properly by replacing the metal. Too many cars like Mustangs, Camaro's and all the other cars that could have been brought back, are only cosmetically repaired, look good from 50 feet away and 6 months to 2 years later they need to be redone properly but it cost to much because now you need to redo the previous work.
I can't impress on the people that are doing this type of repair that your damaging the car more than bringing it back to life...do it properly, (if it takes you an extra year or 2 to save up the funds that's OK) or don't do it at all.