I used to own a supplements company and had my own brands of protein. In my educated opinion, there isn't all that much difference between different brands. Yes, I know, each one has it's special formula and additives, and blah-blah-blah. Believe me, it's more about marketing than science. Don't worry about micellular this and ultra-crossfiltered that. Your body knows what to do when it encounters protein. I also question in most cases, whether the amounts of the additives are sufficient to really affect you. I tend to think not, though of course "aficionados" will argue with me.
Here are the facts you need to know:
whey protein isolate (WPI) - is whey protein that is extra filtered and has a protein value over 90%. It breaks down in your body after about 90 minutes, which is considered to be fast. Some people prefer it for post-workout. On the other hand, the extra filtering removes immunoglobulins.
whey protein concentrate (WPC) - is whey protein that is has a protein value that can vary from 35% to about 85%. It breaks down in your body after 90 minutes. Obviously, WPC is much cheaper to produce the WPI. But since it is not over-filtered, it has some factors that get removed from WPI. Bear in mind, there is no standard for WPC except that it is over 35% protein. Major supplements companies generally do not disclose the quality of the WPC they use.
casein - is a milk protein often blended with WPI and WPC. It breaks down over about 6 hours. It is often used for pre-bedtime protein supplementation because of that.
Also keep in mind that when WPI and WPC are blended, again the supplements companies generally don't specify the ratio. Since WPC is cheaper, there will usually be more of that in the formulae.
With all that in mind, look for a blend of WPI and WPC and shop based on price. Don't worry about the brand so much. I shop based on price, taste, mixability. If it mixes with a spoon and doesn;'t clump I am happy. If you are willing to consider a better option than GNC or Muscletech, visit http://www.trueprotein.com. You can custom design your own blend and they also have many very smart combinations. They have excellent prices and sell other bulk powders also.
However, if you are a brand-slave (much like iPhone users), then, in my opinion, your best choice is Muscle Milk from Cytosport. It is a designer protein combination and has WPI, WPC, casein, and a bunch of other theoretically useful stuff in it. They have lots of flavors and it mixes instantly.