I decided to go with polycarbonate on the grounds that anything I hit that hard I’d rather the force went into breaking the guard rather than possibly damaging my cranks/bearings/bb area. A mate stacked his alu one onto a boulder and twisted the spider on his cranks by a few mm. You could barely see it by eye but you could tell when pedaling or changing gear!
I’ve been through 3 polycarbonate ones in the last 7 years and they have all failed around the bolt holes (usually just one) but never came off or broke in a way that left me unable to pedal (plus I still have a granny ring for real emergencies).
I treat them as a sacrificial and deliberately weak link in a chain of much more expensive parts.