I dont think fiberglass is all that brittle, if you end up with a brittle layup it is probably due to the type of resin or the amount of hardner you used. I have a fiberglass p47 that has suffered a few nose overs on landing and has even flipped over at the end of the runway a few times. It hasnt developed any cracks at all, the main problem is that every time it noses over on landing the fiberglass gets eaten away from friction with the runway. Your main advantage going with kevlar will be overall strenght, but you have to remember to get a fiberglass structure stiff enough for model use it normally ends up being stronger than necessary. Some type of internal structure will go a long way to making the overall structure strong and stiff.