Whey is a complex protein really made up of many smaller protein subfractions, but we don't need to get into all of that right now. Modern whey concentrates contain 70-80% protein, and whey isolates contain a minimum of 90% protein. Now, some may say that an isolate is better. When speaking strictly about protein content, yes isolate is a better choice. Good concentrates contain far higher levels of growth factors (IGF-1, and TFG-1 &2) as well as higher levels or healthy fats and lactose. So don't be under the assumption that a concentrate is crap. They are both good choices and really just depends on the goals of you the consumer.
The difference between Whey and Casein is something you should certainly know and take into account in your diet and supplement regimen. Generally you want to consume some fast acting protein immediately following a workout. Would you take whey protein, or casein protein? In this case, you would want to take the whey, or maybe combine the two. So whats the difference? Mostly its the absorption rate. Both are excellent sources of your essential amino acids, but whey is much faster digesting than casein. Do you mix your whey shakes with milk? Did you know that casein is the most abundant protein in milk? I recommend mixing your srx protein (which has a blend of whey protein isolate and whey protein concentrate) with 8-10oz of milk. This will give you a mix of whey and casein proteins! You're welcome, haha.