Pro has more power. The big projects will need it. I chose a pro when I was doing my iOS stuff as my mate has an air and Xcode just killed his resources when running the app simulator.
That's interesting to know, does the Pro perform well with XCode or is it so-so? I'm interested in looking into iOS dev with the release of Swift so XCode is something else I'll need to keep in mind.
Yeah the pro performs very well for me. If you are wanting to get into Swift there are some awesome beginner tutorials here: http://ios-blog.co.uk/swift-tutorials/
Yes, get the pro. Get the 13'', with 16GB ram, and connect 2 large 27'' monitors.