ABS or fiberglass for hard saddlebags?

ABS will flex better than fibreglass which will start to crackle around any fittings, it will also take a scratch better, paint will last as well on either surface. ABS will also age better – fibreglass will become brittle with age – and is better at impact resistance. Fibreglass has the benefit of being somewhat lighter and cheaper.

Fiberglass Saddlebags

There's far more fairings and saddlebags made from ABS than fiberglass. ABS can crack when stressed or vibration, but generally is not a problem. Fiberglass is good stuff too, however I've found that sometimes they have a very strong odor, especially when new. Nothing nicer than staying somewhere overnight and going to dinner with your wife/date and your clothes stink like fiberglass resin. Kind of ruins the mood. Some fiberglass saddlebags and trunks are coated on the inside to stop the stink.

You're right, ABS is soft, and not suitable. Fibreglass is rigid and takes vibration too.