The following is an excerpt from Knowledge and Nonsense http://maxcondition.com/page.php?103
J. Hale: Casein versus whey. Which one do you like or do you like both? Explain.
Dan Moore: Because I feel that this has been hashed out a million times before, I’ll just briefly say that it doesn’t matter as long as you manage the timing of ingestion accordingly.
Justin Harris: Both have different benefits and faults. Whey is a highly bioavailable protein source with a rapid utilization rate. My only concern with the rapid utilization is whether the body can process that amount of amino acids as quickly as the aminos enter the bloodstream. It will take much longer for the amino acids from 50 grams of protein from a piece of steak to all reach the bloodstream than it will take 50 grams of protein from whey. If those amino acids aren't utilized properly, they can be converted to glucose. If the glucose isn't needed, they can be converted to triglycerides and stored as fat. Casein is digested more slowly. It also is highly bioavailable and has benefits. Casein typically has a poorer macronutrient profile than the whey isolates because casein typically contains fat and sugar. There is a new form of casein that is pre-digested or hydrolyzed that I think will change some people's views on the protein.
Bryan Haycock: I like both. I prefer a "fast protein" pre-workout. For the rest of the day, I usually mix both casein and whey in my drinks.
Martin Berkhan: Casein. It´s long-lasting, anti-catabolic properties make it the ideal protein source for almost all occasions. A mixture of whey and casein would be the best choice postworkout.
Alan Aragon: As it stands today, the research comparing casein versus whey is split right down the middle. And you can definitely predict the outcomes by looking at the funding source of the research. However, one thing’s certain—it can’t hurt to have both types. There might be something to the fact that they exist together in nature so their distinctly different functions would work well together. Whey is more protein synthetic while casein is more anti-proteolytic so their combination would be at the very least complementary. Looking at things optimistically, their combination might have synergistic benefits, but that’s speculation on my part. Speculation over optimal ratios of casein to whey is an exercise in “it doesn’t matter.” This is because for most people, whey and/or casein aren’t going to be their sole protein source for the day. Whether you buy an expensive casein-whey powder or simply throw whey into your favorite dairy whole food, it won’t have any measurable difference. As far as timing goes, again, there aren’t many make-or-break differences. However, many folks relegate casein to the pre-bed period. This might be short changing your options because casein can act as an excellent anticatabolic for the post-exercise period where muscle damage is at its peak.
Layne Norton: I prefer whey. The data showing casein to be superior used whole-body protein balance and amino acid oxidation to determine the benefit of whey or casein. Wholebody protein breakdown and balance are more so influenced by the gut, which turns over by 80 percent in one day rather than the skeletal muscle, which only turns over about <1 percent per day. Thus, changes in whole-body protein metabolism most often is a result of changes in gut protein metabolism. Also, our lab has shown that sustained elevations in plasma amino acids aren’t sufficient to keep protein synthesis elevated over time. We found that even though amino acid levels were still elevated at 3–5 hours post-meal, protein synthesis had become refractory and returned to baseline levels by three hours (Norton, et al). Rennie and colleagues also found that protein synthesis only lasted about two hours in response to a constant infusion of amino acids even though they infused amino acids for about six hours! Thus, it appears that it may be better to use a fast digesting protein and bolus amino acids rather than a protein like casein, which maintains amino acid levels for a longer period of time but doesn’t stimulate synthesis to the same degree as whey. I'd like to end that though by stating that the overwhelming determinant factor will almost always be total protein intake. Worry about getting enough protein in first and then worry about what kind and how to distribute it.