What is the best for a law student - iPad Pro or MacBook Pro?

I would say MacBook Pro. since you are going to be writing a lot, and will need easy access to files and multiple windows opened I would say a MacBook Pro will be a better choice for that.

I'd think for a professional, you'd be better prepared for the future with a MacBook Pro with as big a disk and the most RAM you can afford.

Contact your school and ask them which would be best for your course of study.