Isolate is in general a higher quality protein than concentrate.
From wheyoflife.org:
Q: How Are Whey Protein Concentrate and Whey Protein Isolate Different?
A: Whey protein isolate is the purest form of whey protein and contains between 90 to 95% protein. It contains little (if any) fat or lactose. Whey protein concentrate is available in a number of different types based upon the protein content of the product, which can range between 25%-89%. It will contain some lactose, fat, and minerals. As the protein level increases the amount of lactose decreases. Whey protein concentrate at an 80% protein content is the form most readily available as a protein powder supplement.