Isolates are more pure...they're about 98% protein or so right, but in the procuess of making them its' very easly to destory the protein particle cuz they're so small, i think they make it by seperating the protien particles from the goo thats left over from cottage cheese, well anyway...and the way they make conencrate is when they make cottage cheese right, there is this goo that's left over when the cheese is sepereated from whatever, this goo is dried up and thus you get concentrates, now conencentrates don't get destoryed becuase they're dried up...so you're gettin' about 80% or so protein, but you're gettin all the protein parts. O and as for muscle building, they're both pretty much the same, as long as you get the same amount of protein in, I prefer to consume concentrates just because i know none of the protein was damaged during the process of making it.