Serenity Caldwell, writing for iMore, puts both devices through their paces. In a nutshell:

The Surface Pro and Surface Book are great machines, and I don’t begrudge anyone in the art community for picking one up. If you want something that’s going to act as your primary computer while on the road or at home, the Surface line has a lot going for it. The Pen feels like a Wacom-style tool, and you can get incredibly precise lines and strokes when you write with it. And you can use any desktop apps natively, which can be a huge boon for your workflow.

Where the Surface fails, however, is where the iPad Pro shines. The Pro may not be as powerful but has better battery life, a more natural-feeling stylus, superior entry-level apps, and is an outright joy to draw with. It’s a digital sketchbook and idea machine, and you can hook it up to your desktop Mac using third-party software to get a very close approximation of a true Cintiq drawing experience.

It’ll be interesting to see how reliable the Surface Book is a year or so down the line. As I mentioned in this post, my experience with Windows laptops has been miserable. My experience with iPads has been 100%. I’ve owned 4 different iPads and they are all still up and running.