If you can only have one, buy the Surface 3 if you need a laptop, and occasionally require a tablet. Buy an iPad if you primarily need a tablet and will only once in while need a laptop.
I'm about to go back to school this year (evening MBA program) and actually just finished up a technical degree last year. I usually studied/did research with my MBP in front of me, my Nexus 7 or the family iPad 2 to my left with text books and documents cued up, and a steno book and pen to my right, where I could jot down a quick flowchart or brainstorm. For my upcoming classes, I wanted to permanently ditch paper notes and I didn't want to lug around a 15" laptop. My Nexus 7 was pretty much dead, and my laptop is from 2011, so I bought a Surface 3 to replace both in my workflow. Unfortunately, although the Surface 3 is more than capable of replacing my MBP for non-technical business courses, it can't come close to replacing either of my tablets. Looking back, I was always grabbing either my Nexus 7 or the family iPad 2, and even if I wasn't actually writing my papers on them, I was doing a significant amount of work on those devices. OneNote was my go to app and I was dropping webpages into it, pasting documents, doing simple markups (highlighting), reading my text books on it, etc. My cohort didn't like it, but when it was time for my projects, Asana was my go-to app for project management, integrated with Google Docs. I then used my laptop to stitch everything together and do the other heavy lifting like running VMs and writing light Bash or Python scripts.
My current experience is that apps suck on the Surface. And I don't see that changing any time soon. Yes, I can use a browser or full program to duplicate everything I do from a tablet app, but a well designed, focused app beats a browser or computer version of the same service every time for me. So, I'm going back to the workflow I'm most comfortable with: a 128GB iPad Pro 12.9" tablet with my favorite apps as my primary device, where I do 90% of my work, and my Surface 3 on the backend to finish the product if necessary. And yes, my tablet will be larger than my computer. It's becoming a strange world, indeed.