Classic and No doubt are two complete sets where you can do a full makeup with all the brushes that come with. 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 bit more advanced brushes but is developed 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 is simply a matter of what you yourself like about the type of make-up brushes and which design you like the most :)