Classic and No doubt are two complete sets where you can do a full makeup with all the brushes that come along. No doubt is a slightly lighter collection for a person who is a beginner or does not have a lot of knowledge in makeup. Classic has a little more advanced brushes but is designed to do most of the work for you. Then the Classic collection contains more kabuki brushes suitable for liquid makeup and No Doubt more fluffy brushes. In the end, it's simply about what you think about the type of makeup brushes and what designs you like most :)