I'm a longtime Mac fan, but I'd pick a Surface Pro 4 up over the current iPad Pro. There's a price difference if you go for the crazy-high spec models, but I really like where MS are headed at the moment too.
Apart from the recent OneDrive storage reduction...
My boss thought the same thing....she loved the idea of the Surface Pro.....and returned it last week. Mainly because of the lack of tablet apps (she previously had an iPad, and found that even where there are Windows apps, they are not optimised for touch or lag way behind the iOS version in features). She already has a (Dell) laptop, too, and she said that it felt like "having two laptops, rather than having a laptop and a tablet". She's gone back to iPad (Air), and kept her Dell.