Both those questions are answered clearly in the readme:

compared to avante > I think it's fairly similar. However, magenta.nvim is written in typescript and uses the sdks to implement streaming, which I think makes it more stable. I think the main advantage is the architecture is very clean so it should be easy to extend the functionality. Between typescript, sdks and the architecture, I think my velocity is pretty high. I haven't used avante in a while so I'm not sure how close I got feature-wise, but it should be fairly close, and only after a couple of weeks of development time.

And:

Another thing that's probably glaringly missing is model selection and customization of keymappings, etc...