It’s hard to imagine many people shopping the iMac Pro against the Surface Studio. If you want a large tilt-able screen to draw on, your only choice is to buy the Surface Studio. On the other hand, if you want the power of a workstation in an all-in-one form factor, your only choice is to buy the iMac Pro. If you are considering the Surface Studio at all, then you don’t need the power of the iMac Pro, so you’d be looking at one of the standard iMacs, not the Pro.
The only reason why someone might be considering the iMac Pro and the Surface Studio at the same time is if they simply like buying cool tech and how the tech is going to be used isn’t much of a factor.