Over the past few years, the iPad line has gotten quite complex. We went from the iPad, to the iPad mini, to the iPad Air, iPad Air 2, iPad mini, iPad mini 2, and iPad mini 3. They also range in storage size from 16 GB to 128 GB. When schools are looking to deploy a 1:1 iPad program, they are left with a lot of options. The question comes down to which do they pick.
The first generation iPad mini was a really exciting machine when it was originally released, but its technology has dated quickly. It’s actually an iPad 2 shrunk down in size. This means that it’s 2011 technology that you’d be deploying for the 2015–2016 school year. Apple has just recently stopped selling it, but it's likely still available through resellers for the time being.
iPad mini 2The iPad mini 2 will be two years old this fall, but it’s still a capable machine. It includes Apple’s A7 processor, and it’s 64 bit. It has 802.11n Wi-Fi.
iPad mini 3The iPad mini 3 is the current iPad mini, but it’s not really that different from the iPad mini 2. The only difference is that it comes with a gold color option and includes Touch ID.
iPad AirThis iPad is a generation behind currently, but it is still extremely powerful. It’s a full size iPad, and runs Apple’s A7 64 bit processor. Because it’s a full size iPad, you’ll have more room for content.
iPad Air 2This is currently the fastest iPad on the market. It’s running Apple’s A8X chip. It also contains the best camera out of any iPad (8MP vs 5MP in the rest). Like the iPad 3 mini, it contains Touch ID as well. It’s also the only iPad with 802.11ac Wi-Fi support. iPad Air 2 will also be the only iPad that gets the iOS 9 multi-tasking upgrade.
What's the best?

Right off the bat, I would eliminate the iPad mini 1 and the iPad mini 3. The iPad mini 1 is just too slow to rollout for a large deployment. The iPad mini 3 isn’t enough of an upgrade of the mini 2 that I’d recommend paying the newer model.
All of the ones remaining are in no way slacking on technical specs, but with the faster processor, better camera, and 802.11ac support, the iPad Air 2 is obviously the best iPad on the market. When it comes to deploying hundreds if not thousands of iPads, I’d probably recommend (budget wise) that you stick with the iPad Air or iPad mini 2. They will likely be supported software-wise for years to come since Apple is still selling them. You’ll save a considerable amount on your lease/purchase price by opting for the older model, but still end up with a very fast device.
However, I do recommend that 1:1 deployments go with the full size iPad. The larger screen size is much needed when it’s a primary computing device.
StorageSizeA lot of schools try to save a little money by keeping the storage size at 16GB, but I highly recommend you get the 32GB model (the middle tier). It’s a small price to pay for a 2x upgrade in space. Apps and books are not getting smaller, and a 16GB device will cause additional help desk time for students wondering how to free up space on their device.