"The processing of carbon fibers produces three types of fiber: "High Modulus" fibers with marginal strength and marginal elongation to failure, "Intermediate Modulus, Intermediate Strength" fibers with higher elongation to failure, and "High Strength" fibers with marginal modulus and marginal elongation to failure."

Im6: 760ksi tinsel strength 40 millionths modulous

Im7: 780 ksi tinsel strength 40 millionths

Im8 790 ksi tinsel strength 44 millionths modulous


No material is better than another just some materials are stronger than others and some are stiffer than others. still each material should be used according to it's strengths.
Fot instance if you want a " softer " rod you might choose im6 instead of Im8 or if you wanted a stiffer rod you might choose the IM8

Or maybe you want a rod thats stiff in the tip but light and a but section that loads well you might make the tip out of im8 and the butt of im6..
there is no quality difference between im6 and im8 or any other material just different charecteristics and therefor different applications..