When comparing Material UI vs MUI CSS, the Slant community recommends Material UI for most people. In the question“What are the best frameworks/libraries for implementing material design on the web?” Material UI is ranked 4th while MUI CSS is ranked 5th. The most important reason people chose Material UI is:
Really nice implementation so far
Ranked in these QuestionsQuestion Ranking
Pros
Pro Adheres really well to the material design standard
Really nice implementation so far
Pro Perfect for React projects
Really convenient and easy to use components

Pro Fast
Simple animations without too much fanciness keep the performance high and size small.

Pro Lightweight
11KB minified and gzipped

Pro No external dependencies

Pro Grid system - the same as Bootstrap
12-column grid

Pro WebComponents library

Pro HTML Email library

Pro React library

Pro Customizable using SASS
The MUI CSS can be easily customized by using the SASS files available on GitHub and via Bower. Customize breakpoints, font-settings and use Material Design colors.
Cons
Con Active development means API changes are frequent
Might be a problem for large projects, but since active development is a good thing, you should consider this rather mild inconvenience weighed against the benefits.
