That very much depends on what your interests are. Both MEAN Stack and Machine Learning are very popular fields. Machine Learning is more based on working with raw data, cleaning, transforming, normalizing it, and then applying algorithms to teach machines to do something like classifying an image or predicting the weather etc.
I would suggest that you try both out and then pursue which one you feel more comfortable with.
However if you wish to lean MEAN Stack i would suggest you start by learning individual technologies of the stack like MongoDB, Angular etc.
You can also read this blog that explains what MEAN stack is.
If you wish to learn MongoDB you can do this end to end certification course on MongoDB.
Also you can learn Angular by doing this Angular course.