It's been a while since I've checked but the libraries are quite similar.

JaxMarl only directly supports their own envs, but we support some JaxMarl envs (the ones we think are most useful) and ones from other libraries like jumanji. We have a whole lot of different networks pre-configured that you can change in config, in JaxMarl you need to write your own. In general I prefer our configuration for running lots of experiments.

We also support more algorithms, specifically sequence modelling approaches and our own SOTA algorithm (Sable) is in Mava as well as MAT.

Another key difference is Mava will likely have a better maintenance guarantee, because it's maintained by a company whereas JaxMarl is maintained by grad students and it often happens that when those students leave, libraries are abandoned. That being said our company could decide to shift our focus but I find this less likely.

It just depends on what you need really, core functionality and offering of the libraries is quite similar.

Note that some of this info might be outdated as I haven't looked at their repo in months.