My 14 yr old wears the AMX5 as well. We couldn't justify the difference in price, knowing he would only be able to wear them a season or two. We have been very pleased with the protection and comfort. He's bailed a couple of times (looped out on jumps) and I just knew his knees were goners. But, the braces did as advertised.
Some insurance companies will pay for the braces (mine won't). I know CTi is good about helping out in this area, and the others may as well. The AMX5 is about $270.00 for a pair. The other guys are twice as much or more. The look is very low tech compared to the higher priced versions, but nobody sees them anyway. I am interested in finding out more about the "crumple zone" though. If the force is great enough, something has to give.
I don't let him ride without them.