While whey protein appears to be more potent at stimulating protein synthesis (68% above baseline by whey, 31% by casein) whey protein fails to inhibit protein breakdown while casein can reduce protein oxidation by 31%.  The net result is either similar accumulations in muscle tissue content after ingestion of equal amounts of whey or casein protein,  or better retention with casein over 7 hours if nothing else is ingested;  this is despite 30% greater leucine uptake into muscle cells when measured at 2 hours post ingestion relative to casein .  One study that divided protein synthesis rates into a 60-210 minute period and a 210-360 minute period noted that whey increased protein synthesis significantly more than casein in the first period only, with the opposite result occurring in the latter period and no overall difference over the 5 hours tested, although casein trended towards being more effective. 
Many sports scientists, however, say the compounds are a viable alternative to dangerous performance-enhancing drugs. They point to the fact that these compounds are isolated from natural food sources, and are therefore no different, ethically, from high-carbohydrate food mixes for aerobic athletes. Besides, they say research has shown them to be medically safe and that we should emphasize that the alternative to these natural compounds is steroids.