Maybe I'm typing for the sake of reading my own content at this point, 14 pages into this discussion.... But I've owned a Surface Pro 4 that I used daily for work, as well as an iPad Pro 9.7" with Apple Pencil and Apple keyboard cover.
After plenty of time working with both devices (as well as a Macbook Pro 15" laptop in the mix), I'd summarize the whole situation this way:
- Credit where credit is due. Microsoft did a pretty amazing job packing a full blown Windows PC into a tablet form-factor. How well it performs depends a whole lot on how much you spent to get a better configuration (just like with Apple products). I would pretty strongly dislike a Surface Pro 4 with the basic "M" series processor and only 128GB of SSD storage in it! The one I used had 16GB of RAM, a 512GB SSD and Core i5 CPU in it. Configured that way, or even with 8GB of RAM and 256GB SSD, you can easily forget it's not a full blown Windows 10 desktop PC when you use it with the dock and a normal monitor, keyboard and mouse.
- When you're on the go, things drastically change. I find I actually prefer my iPad Pro because it's a "pure" tablet. Yes, it's not going to give you apps quite as powerful as ones intended to run on a regular PC or Mac. But everything on the iPad is designed to work specifically on an iOS device, and is aware of the screen resolution and touchscreen nature of it. Windows 10, by contrast, always feels clumsy on the Surface Pro because your experience varies depending on what you run. Metro UI based apps made for Windows 10 may give a great tablet experience, fully supporting use of the pen as an input device, etc. But other big name Windows apps aren't aware they're not on a standard PC at all, and may not scale well to the high DPI screen. In some cases, you even end up installing duplicate software -- one to do a task in the Metro UI and another that does the same basic thing except as a regular PC Windows application. I think most Surface Pro users, on the go, wind up using it just like a traditional laptop with the keyboard cover. That's a compromise in and of itself, since that keyboard kind of sucks to type on, and is too flimsy to hold the tablet upright while trying to use it in your lap.
- I don't view Microsoft's latest Surface Pro update as anything more than an incremental improvement on the SP4? It seems like any claims Apple tried to "copy it" are just marketing B.S. - because both companies announced relatively incremental changes to existing devices at about the right time in the product life-cycle.
I should probably clarify that I'm not an artist and my use of the digital pen/pencil is limited to using it to jot down short notes, annotate documents, or fill out digital forms. I think if you really are one of the "creative pros" who plans to do a lot of digital artwork on a device, you approach the entire thing from a different angle than the rest of us. In that situation, I think I'd go with whichever tablet has the drawing apps on it you're most comfortable using, and/or which pencil feels more comfortable and functional to you? My own conclusion was that I'd rather keep the iPad as a "secondary portable device" to use when appropriate, combined with a standard laptop for everything else.