That honestly depends on what your intentions are. Here's the good and the bad from a guy who's run the gamut:
* Older iPads (post Air 1) work perfectly with all DJI apps. You will notice little or no difference in this regard.
* SOME APPS DO require more horse power. So if you intend to do COMPLEX automated flights, I recommend the Air Pro or better.
* Newer iPads can't be hacked as easily as the old iPads can. This limits you to using the iPad the way APPLE tells you and the way DJI TELLS you. Starting with Gen 3 (easily spotted via the USB-C interface and missing 1/8" audio jack), is much more difficult to hack.
* Hacking is required for system reliability and control (I'll site the almost daily myriad of fly-aways, crashes and loss of control seen in ALL 3 "Pilots" forums). In 6 years of droning, I've had none of that.
* Hacking is required to avoid random, erroneous login requirements to agree to new terms or takes tests or lord knows what. These "tests" and login requirements DISABLE YOUR DRONE until you find Internet access, login and take the GD test. If you're a casual flyer who only flies in-city for phun, this is an inconvenience. If you're a professional who flies out on location (99% of my work), then these login requirements are an absolute deal-breaker.
In my mind, the Air Pro, Gen 2 (with Lightning interface and 1/8" audio jack) is the sweet spot. It's the latest iPad that is still hackable, and yet has enough horsepower to run complex automation tasks.
D