I can't comment on the iPad Pro, but I've been using a Surface Pro 4 exclusively (aside from my phone) for over a year now. My office is converting everyone to the Surface Pro as our primary device (no more laptops or desktops). I've been very happy w/the SP4. I'm a lawyer, so I do "lawyer-type stuff" on it, which is to say that I'm not working w/accounting software, spreadsheets, etc. The SP4 connects easily to my two monitors. The only issue I've experienced is using the two monitors together and trying to access .pdf documents. The resolution and size is wonky, i.e. HUGE. Our IT folks resolved my issue by uploading a different reader and tweaking, but since I don't speak IT I can't really explain what they did. It was not an issue when hooking up to only one monitor.
I really like that I can use the SP4 as a tablet or w/the keyboard. Convenient. I also like that I can just pop it into my backpack and take it back and forth from home to the office. It has many features that I haven't even tried yet, so this review is a bit thin. Overall, it is a great option and I much prefer it to my old laptop. The only thing I occasionally miss is not having a disk drive, but I just plug in an external. I also use a "spider" attachment to give me additional USB ports when I need them.
(oh, and ditto
@JeffB 's comment re the difficulty of using the SP4 as a "laptop." It isn't a lounge-on-the-couch-as-you-work device unless you've got terrific balance or like a challenge. All it takes is one grand lap-leap from the cat and the device is on the floor).
(oh, again, the sound quality on the SP4 is great! It was our principal sound machine in the condo on last year's Taos trip.
)
Hope this helps!