I'm looking to pick up an iPad mostly for business use (email, reading and editing docs, taking notes, etc). When I travel, I will of course use it for movies and other entertainment as well, but that's secondary.
Without getting too deep into the retina complaints, which do business users recommend?
Also, if it makes a difference, I own an Air and iPhone 5
Oddly enough I'm in almost exactly the same situation with the same hardware though I've also got the iPad 3 for home use which has been pressed into work duties in the past and for me the choice was easy: iPad Mini every time.
First up ignore the retina display purists. It's a lovely, lovely thing to have but it's NOT essential. To give you some context the Pixels Per Inch figures for your devices are 135 for the 11" Air (128 for the 13") and 326 for the iPhone. The Mini comes in at 163 so if you're happy using your air on your lap you'll be fine with the Mini.
I'm assuming as you have an air you're looking for portability as one of your prime requirements and it's here that the Mini will be so much better than its big brother. While the iPad is by no means heavy you will notice the weight in daily use but it's the size that's the killer. I can just about squeeze mine into the map pocket of my waterproof but it's hardly ideal and rather uncomfortable. Yes, you'll sacrifice a little size and I'd expect some apps to be a little harder to use with smaller touch targets but it's a tradeoff worth making if portability is important.
As for editing documents do yourself a favour and budget for a bluetooth keyboard and iPad stand. You'll know from your iPhone how well you get on with the on-screen keyboard but, in my experience, for anything other than quick edits a full-size proper keyboard is a huge bonus. Leave the iPad bluetooth turned on and just power up the keyboard when you need it.
In my case my main requirements where 1) to be able to either carry around locally or remotely access all document and e-mail, 2) to be able to take digital notes in a meeting and keep them organised by project and 3) to access web-based software systems all in a device that would just replace my normal notebook and fit in a pocket (primarily for dashing between buildings when it's raining). There's also the usual criteria about having enough flexibility in the device and choice of apps in the ecosystem to further adapt workflows in the future but that's normal. Considered an iPad 4 but the smaller form factor and much lighter weight of the Mini along with reportedly superb battery life swung it. When I'm at my desk I've got a nice Griffin stand (literally just a stand, no built in charging or anything) to drop the iPad on and an Apple bluetooth keyboard close by.