In the average typical American the diet consists of 2-3 times more protein then it should and this is without protein supplements. To build muscle you need to be exercising. Commonly, people do not think that excess protein can be stored in fat, but it can. The protein is not stored ``as fat`` but rather ``in fat``. If you are not using the protein it will be stored in fat cells. Excess protein does not get stored in muscle.
So if you buy the expensive stuff or the cheap stuff your still wasting money since your getting enough protein in your normal diet already.
Additional:
providing the information that you are a body builder does affect my answer. The average person is not a body builder Mr. Coleman(or Mr. Mackenzie) however this does not affect the answer very much and my original answer still holds true for the average person that is just working out. The amount of protein that a body builder needs is dependent on their muscle mass in order to maintain the already formed muscle. Your calorie consumption is also very high in order to maintain your muscle. Since you are using and building muscle your body is using the protein to make muscle rather then storing it in fat cells.
To answer your question since you are set in having protein supplements is that Whey protein isolate contains a higher percentage of protein(almost 90% protein). Many top athletes choose whey isolate rather than normal whey.
Thus, your second question, the higher protein percentage would be better for growth providing your putting the effort forth to form it.