Re Surface 3 vs Surface Pro 3, I tried an Atom processor-based Lenovo Tablet 2 just to see how I liked working with a current-technology Windows tablet. What I found was that the Atom was barely adequate for even just web browsing. Slow scrolling, missed gestures, etc. Since I wanted a single Windows machine for everything I do, this low performance level was just not acceptable.
In the Surfact Pro 3 I found benchmarks that put the CPU performance of the i5 version SP3 at about 80% of the performance of the i7 but with, of course, somewhat better battery life. From that I decided that the i5 would be adequate for my needs and that has worked out just fine.
Re size, I would like to have something closer to an 8" screen with a really thin bezel, giving an overal size similar to the Nexus 7. I find that to be a good size to use when traveling. In the cockpit I suction-cup the N7 to the side window, which puts it pretty much right under my nose. In that position the smaller screen is actually bigger for me than, say, an iPad on my knee. That's why I waited for a couple of years for a small Windows tablet to appear before accepting the larger SP3.