Casein is best before bed, yes. It is slow releasing, and will help with recovery. Note that protein does not make lose fat, per say. You might use this protein as a replacement for your nightly cravings, which would be a wise choice.
Whey protein isolate or concentrate is typically used during the day, after a bout of exercise. Again, it is a wise option because a good whey will have no added sugar, and is a healthier way of getting a recovery snack into your diet, and boost daily proteins to encourage recovery for muscle tissue.
Both are good options to help maintain muscle. For real success pair up one or both, using 1 scoop of whey after training, plus one scoop of casein right before bed. Combine this with a lower carb diet- cut out all processed foods, and use a solid strength training program 3 to 4 days per week.