Okay, so it's not a picture of the real things, but it's a good illustration of the size difference of the actual screens. It's easy to come by the dimensions of the tablets themselves, but finding stats on just the screen is a little harder. Since the aspect ratios are different (4:3 on the iPad Pro vs 3:2 on the Surfacebook) comparing inches to inches diagonally doesn't tell you which will be wider in portrait mode, which is generally what matters most for music reading.
Thankfully, there are forums full of people willing to do the math for me and website that will nicely line up these little boxes. Thanks to Marty on the tabletpcreview forums we can see how the screens actually compare.
Even though the IPP has less area (79.88 in² vs 84.12 in²), it's got .3" in more vertical landscape space (7.74" vs 7.49")For more reference, the surface pro 3 I use now is about 6.6" and the 3rd gen iPad is 5.75". At nearly an inch wider, the Surface Pro feels roomier when reading music. The Surfacebook would give me nearly another inch, and the iPad Pro would give me a little than an inch extra. I'm not sure the .3" difference between the two would really make much difference. The aspect ratio only really comes into play if you've scanned A4 orchestral music at full size. Then the extra height of the Surfacebook would come in handy. Most of what I read is in 8.5x11 PDFs, so it's a wash. Either would make a good upgrade.