Which one to get? I'd say it depends on what is more important to you. Based on what little information we have, my feeling is that if you want a really good tablet experience, get the iPad Pro. If you want a good laptop experience, get the Surface Pro 4 or Surface Book. Yeah, the SP4/SB can be used as a tablet, but the ecosystem is lacking. Yes, you can try to use the iPad Pro as a laptop, but the ecosystem is lacking and iOS makes certain tasks very difficult or outright impossible.
I tried a SP3. I really wanted to like it, but as a tablet it was pretty subpar compared to a regular iPad. The pen was also quite disappointing from a drawing perspective. A very noticeable downgrade compared to my old Intuos 4 and I'm not even talking about the pressure sensitivity levels that people always bring up. I'm talking about the fact that the SP3 pen can't draw straight diagonal lines if you go below a certain speed. It just jitters all over the place. Known problem but not often talked about.
To me, it boils down to which one offers the best pen drawing and PDF experience, because that's what I would use it for. I already have great devices fulfilling all my other needs. I don't need a Swiss Army knife. I'm leaning towards the iPad Pro for what I want. But if I was looking for just one device that can do it all and be okay with it not being the best at everything, then I'd go for the Surface Book (or SP4 if on a budget). I'll wait for the reviews to show up before I make any decisions though. We have almost no information about the SP4/SB and only a spec sheet for the iPad Pro. There are so many crucial missing details at the moment, so it's way too early to make any final judgement.