Yves Boire is your man for the science behind this:
http://www.brothersofiron.com/showthread.php?t=1726
Long story short - whey protein is digested almost immediately, and raises your rate of protein synthesis. This is why you should take it immediately after exercising. However, after about four hours, your protein synth rate drops to BELOW what it normally is.
To counter this, drink casein within 2-4 hours after the whey. Casein digests more slowly, and it'll raise your protein synth rate much more modestly than whey, but will keep the rate from dropping below what it normally is. Casein is also very good to take right before you sleep, since your body sometimes breaks down muscle tissue to repair the damage of your workout.
I don't know much about the differences between brands. I don't think it matters THAT much as long as you get something functional, since most brands exaggerate what their product can do anyway. I find even the stuff at Trader Joes works well, and is very cheap. The main thing is you have to put in the time to regularly exercise - whey/casein won't build muscle, they'll just repair it faster. If you can only afford one I'd say go with casein, drink it every night before going to bed, and make sure to eat a decent portion of protein immediately after your workout.