There is a good chunk of overlap but mise predominately pulls from github releases artifacts/assets and pixi uses conda packages. While mise can use conda packages, the mise-conda backend is still experimental. I don't think github releases or conda packages are better than the other, they both have tradeoffs.

Pixi is very python focused, it's both a tool manager and a library dependency manager (see uv/pip). Mise considered library dependency an anti-goal for a long time, while I don't see that on the website anymore I haven't seen any movement to go into that space.