I'm about to get one as my home phone. Already been using one from work for a few months now.
My honest opinion: not only is it a lot faster than the Note9, it has better reception, is much easier to hold, has a dramatically better camera, offers faster face unlock, has been getting updates faster. The UI is perfectly fine (and much more lightweight), if you don't like it, replace it with a different launcher.
I absolutely LOVE the Mate 20 Pro and can wholeheartedly recommend it. If you value camera over pen, it's an absolute no-brainer. It's by far the best Android phone available right now, and likely still will be after the S10. The Kirin processor is pure insanity speed-wise.
I don't dislike the Note 9 per se, but it's heavy and the cameras are simply underwhelming