For those thinking maybe they should go for the Surface Pro/Book line, you should pay attention to the enormously broad swath of complaints levied at its terrible production quality and numerous problems across the board.
I have all the reasons one could think of to choose one of those devices over iPad Pro but I can not in good conscience do so. Besides the in-person experience being rather poor (heavy, low quality construction) there are numerous software/firmware/hardware issues being reported all over the place, reviews on every site, enthusiast forums like /r/surface, and so on. Thing is, this is the same occurrence on every Surface product launch from the very beginning, except this year seems substantially higher volume. Some are speculating it's due to Microsoft downsizing massively their testing department last year. I don't know. But I did monitor the complaint rate on a few enthusiast forums back when I was considering the SP3. That rate is absolutely exploded. It is not "par for the course" for normal ambient complaining. There are serious flaws in these products.
And why should that surprise you? Whey else are you thinking about iPad Pro? The iOS lineup is immensely stable and well performing compared to all other platforms, that's the #1 reason to even consider it for non-phone uses at this point, I think.
So if you really do think the Surface Pro/Surface Book is the better fit, be prepared to deal with Windows hell and half baked hardware designs. Considering how livid people get around here at the slightest flaws with iPads at launch, I can't even imagine those same people using, say, the Surface Book (which has a tablet the same size as iPad Pro) what with the properly produced models experiencing creaking cases and LCD ripple along the sides when held normally, and so on.
It's true, the Surface options have more power, and run desktop software, and maybe there are specific niches that they serve better, but, it's always a sub-par experience with software not optimized for touch or only barely so.
In reality, the iPad Pro is a new product category and it is going to take time for software to show up that properly serves it. Or maybe it never will. But it's not the same product as the Surface line. Those are windows notebooks in slightly newish form factors and they include every single windows laptop cliche problem you could imagine.
Personally, I'm still weighing the decision. Surface Book for $2,100 (i7, 8GB ram, 256gb storage) vs. iPad Pro top end with cellular for 1079. Different devices, yes. Pros and cons in each for me. Legitimate reasons to choose either. But right now the biggest problem I have here is the instability of Surface products that is only worse now. Tough call!