The Surface Pro is the better machine if your requirements are for it to run Windows and lots of Windows apps. I chose the iPad because its OS is designed from the ground up for portability and finger prodding, all the apps are designed for this use case in mind too. I've used Windows 10 in tablet mode (I had a HP X360 for a while) and although it wasn't awful, it wasn't great either. I'm much more comfortable getting stuff done on the train, or sitting watching the TV, on the iPad Pro.
BTE, I'm hardly an Apple fanboy as I do have a 5820 based PC for gaming and only got my first Mac in 2014. But now that Office has been updated for Mac, there's very little in terms of productivity that can't be done on an iMac, unless you need MS Project or Visio of course...