iPad has always had 4:3. And it's still quite a difference between the two, 3:2 still feels very strange when you hold it in portrait. Holding A4 paper in portrait makes this even more obvious.

You are right, iPads had always 4:3, mea culpa.

"The aspect ratio of A4, 1:1.414 is near the average of 1.5 for 2:3 and 1.333 for 3:4. US Letter is 1.294.

Still Surface Pro 3 aspect is better than 16:9 (1.78) at least.

Yep. Now add some buttons while viewing an A4 paper. On an iPad they fit nicely next to where you're holding it, on a Surface you either have to put them in an awkward position, shrink the content or have them it popup in some generally non discoverable gesture.