I wouldn't be surprised if most people can't notice any differences in stiffness between two cranks. Even more so with mountain bikes that have fairly wobbly frames (compared to a hardtail), wobbly wheels, suspension, etc. etc. Could be wrong, but it would be interesting to see how many (few) people could actually notice it without it being pointed out (for example comparing two XTR-like cranks that have different levels of internal milling and look the same on the outside). Why go with an alloy model? At the face of it I can only point to two factors. Price and impact...