Cathinones are, strictly speaking, a subset of amphetamines - however, the subtle differences between them warrant this sub-family to be treated as a class of its own.
There is a surprising phenomenon, when it comes to molecules - that small changes to molecules sometimes create very profound changes in effect, This tendency to reliably contradict our intuition - is called the QSAR Paradox. This paradox is part of a larger problem found in the determination of molecular similarity in chemistry.
Richard Glennon studied the QSAR between cathinones and amphetamines and published his findings. In the video on this blog post, I compare 4-MMC to 3-MMC as an answer to the question: "what is the difference between the two". This difference is a very good example of the way the QSAR paradox confounds our understanding of how small differences in molecular structure create profound differences in molecular activity.