If you're referring to scientific results, here's a study from the JSSM
"Protein, which is best?"
http://www.jssm.org/vol3/n3/2/v3n3-2pdf.pdf
Pay particular attention to "table 1," which shows the difference in PER, BV, and net utilization.
Under conclusion, you'll see the article says:
"In athletes supplementing their diets with additional protein, casein has been shown to provide the greatest benefit for increases in protein synthesis for a prolonged duration. However, whey protein has a greater initial benefit for protein synthesis. These differences are related to their rates of absorption. It
is likely a combination of the two could be beneficial, or smaller but more frequent ingestion of whey protein could prove to be of more value. "
Key points from the article, in case you don't want to read the entire thing (I didn't):
KEY POINTS
• Higher protein needs are seen in athletic populations.
• Animal proteins is an important source of protein, however potential health concerns do exist from a diet of protein consumed from
primarily animal sources.
• With a proper combination of sources, vegetable proteins may provide similar benefits as protein from animal sources.
• Casein protein supplementation may provide the greatest benefit for increases in protein synthesis for a prolonged duration
Proof enough, from a reputable source (mind you), for you?