What iPad for the cockpit?
In my experience, 4G is not reliable above 1,000 feet, and is frequently non-existent above 3,000 feet.

Many pilots have suggested the 4G model for two key reasons:


The 4G model has a GPS-chip in it, that the WiFi-only version does not.
Having a cellular connection means you can do on-the-ground operations in places that don't have WiFi. This includes things like: File Flight Plans, Check NOTAMS, and update maps and charts


Personally, my iPad is WiFi only, and I pair with an external GPS receiver for great accuracy.

I use the "regular" iPad (9.7"), but that is a little bulky in the cockpit of a 172.  The Mini may be preferable if space is at a premium.