I think to compare an iPad Pro to an SP3 doesn't make sense. A tablet running a mobile OS is mainly for media consumption and web browsing. A hybrid like the SP3 is meant to work as a media consumption device as well as one for real productivity. So someone who feels they do not require the form factor of the SP3 for productivity would consider owning a ultrabook and iPad Pro. I tell people all the time that the SP3 isn't the best tablet experience or the best laptop experience. But it is a happy medium. I'm a photographer so I use my SP3 to run Photoshop CC and LR5. I like not having to travel with two different devices or having to pull each one out for different situations.
iPad Pro may run iOS but it's premature to assume that it does.