Whey and casein are both complete proteins that are beneficial for those looking to bulk up. But they differ greatly when it comes to digestion rate, muscle growth, and use.
As this video from the PictureFit YouTube channel explains, whey is a faster acting protein. It reaches the bloodstream quicker and elevates blood amino acid levels much higher than Casein. It also has more Leucine, which helps you build more muscle. Casein is slower to reach the bloodstream and releases smaller amounts of protein over time. But that also means it stays in the bloodstream for much longer -- about eight hours compared to whey's three. It also has more muscle preserving properties than whey.
Regardless of what protein supplement you choose, the most important thing is your overall protein intake. So neither is truly better than the other as long as your reaching your protein consumption goals. That said, whey is better for protein synthesis and growth, is usually easier to consume, tastes better, and is cheaper. But casein is better for preventing muscle protein breakdown, so a combination of both is ideal. Use whey protein supplements post workout, and use casein before you go to sleep.
WHEY vs CASEIN [YouTube]