from my experience the 'duraflex' type gear just ends up losing shape a bit easier out of the moulds and is also generally thinner. there may be some that are better but as far as i am aware its still based on a fibreglass it just has different additives. i bought some of it once and it didnt fit anywhere near as well as my standard fibreglass bits.
yes fibreglass is brittle, but good strong fibreglass can crack and easily be repaired and should fit okay. but this stuff tends to be iffy to begin with. Plus if you hit it hard enough and it flexes that much and doesnt crack itself, it will likely still crack or 'craze' the paint anyway