The main difference is Jax is for python primarily, while MatX is c++. This might seem like a poor answer, but in many domains (quasi-real time, signal processing, etc) the language is important to give certainty guarantees on performance.

MatX has been used in several projects with hundreds of microsecond deadlines, which is not usually something you'd choose Python for out of the box.

We are instead targeting users who already have python or high-level code that they need to port to c++ for whatever reason, and want to do it in the easiest way possible. With c++17 we're able to provide a simple syntax without compromising on performance compared to most native code.