One of the first decisions you'll have to make is do you need a cellular data connection. iPads come in two versions, WiFi only and WiFi plus cellular. WiFi plus cellular iPads also have built in GPS capability which means they can use navigation apps even when offline. The newest iPad Air 2 offers the fastest processor currently offered in any iOS device and comes with 2GB of RAM. Both the iPad Air 2 and the iPad Mini 3 offer Touch ID.
The iPad Air 2 and the iPad Mini 3 offer storage options of 16, 64, and 128GB. The iPad Mini, being smaller, is more portable but doesn't have the faster A8X chip found in the iPad Air 2.