The iPad Pro isn't going to replace a laptop. They can say all they want it can/will but it won't. It's just a larger iPad, other than the Apple Pencil there are no new features and the Pencil is only going to help artists. I have a rMBP, there is no way I would not use that for creating something over the iPad Pro. I still bought one, I want a larger iPad. If Apple was serious about the Pro replacing laptops it would have 256gb of storage or more available. At the very least it needs an SD card slot, but we all know how likely it is that ever happens. lol.
The problem with the Surface 4 is it's not great at being a tablet because, 3-4 years into Windows tablets there still aren't a ton of apps. Which means it's more of a very expensive laptop with a horrible and expensive keyboard, that can be used in tablet mode. And it runs Windows, which gets worse with every version. For the price of a Surface 4 with the keyboard, or the Surface Pro 4 with a keyboard, you can get a superior laptop. There are tradeoffs in everything no matter if it's running OS X, iOS, Windows, or Android.
And I would skip the Apple keyboard for the iPad Pro. If you need/want a keyboard look at the Logitech one. I've had Logitech keyboards for iPads and the Logitech Pro for the Samsung Note Pro and they are great keyboards. They're not as thin as a smart cover like the Apple keyboard but every review I've read so far of the Apple keyboard is highly critical.