I would like to keep this from getting too political*, but cats is for all intents and purposes scalaz. It has not reached full parity as of yet, but keep in mind it was only created a few months ago. The goal is for it to be a more pragmatic approach and more democratic when it comes to its evolution. So, naming of operators and classes is hopefully going to be a little more straightforward, as well as it has no qualms with using mutable data within a method if it means better performance. Last, they are HOPING to have better documentation….all of this means that it may end up becoming a replacement for scalaz with a better beginner’s approach for those not embroiled in the math world. If you want a fuller answer, then maybe head over to their gitter board and Erik (non) could answer it himself 🙂
*The gist is that scalaz has some social baggage with it that causes a number of big names to shy away from using and/or contributing.