You don't need cellular connectivity on any device, but it's a nice to have for two reasons:
- It can load the background map/satellite imagery when you have no WiFi coverage. This requires a data plan.
- It can use the iPad's GPS for extra features such as follow-me mode. This does not require a data plan -- you get GPS if you have the cellular hardware on the device.
There's really no difference between the iPad and iPad Mini other than size. But, keep in mind that the DJI Go app uses battery relatively quickly, and requires a fair amount of processing power. My iPad
Mini 2 is currently fine, but my older iPad Retina won't even run the app-- not enough horsepower. To future-proof yourself, I'd get a newer model (Mini 3 or 4).
Please fly safely and considerately!