Also isolate is more important to some people than others. One person may need isolate to get fast bioavailability, and the next may get comparable results from both isolate and concentrate. If you don't care about the cash, just use isolate. If you do, experment with both to learn how much better (if any) isolate is than concentrate for you. Then you'll know how much difference in results you're getting for the extra $$$.
Someday there may be a test to see whether you're a person who needs isolate or not. Until then it's trial and error.