MPhil is a "better" degree than MRes, meaning its of higher regard. If it's you're worried about the polishedness of a course, you can always look at lecturers and academics that teach. In my most humble opinion, go read MPhil in Cambridge (I don't have any experience in any) but I guess you don't have to do a job your degree is in. And carrying that Cambridge name will be beneficial.
If Imperial is cheaper and thats what your big concern is, study there instead. If you want to do a PhD in Machine Learning, you can probably do that with the MPhil from Cambridge. I recently heard that Imperial is second on the QS rankings. So you are not wrong with either choice. Just depends if you'd like to move out or not, I don't think the course here matters as much.
This is all my opinion, and others should probably chime in before you make a proper informed decision.