In general, any differences in ride quality are almost wholly determined by the engineering and not the material. For instance, people quote aluminum as "harsh riding" and yet, if I give you two 3/4" tubes of aluminum and steel with the same .025" wall thickness, it will be easier for you to bend the aluminum over your knee.

So what makes the difference?

Simple... it's the combination of material, tube sizing and even the alloy. Take aluminum... they typically use 6061-T6 and 7005-T6. Since the 6061 is about half as strong, they use 50% larger tubes for it. Now, here's the kicker, while the 7005 is about the same strength as 4130 CrMo tubing, it is also more prone to stress fractures. So they overbuild a 7005 bike with larger tubes to prevent catastrophic failure. Some people characterize this as a harsh ride, when in fact it is designed to be just as durable.

IOW, the only way to tell is to ride the frame and decided after.

:)ensen.