Hi John -
Thanks so much for the kind message :)
So the advantages Peter mentioned apply to all of the hardware models (even back to ipad 2). The "hardware acceleration" simply takes advantage of the DSP located on every ipad to do faster computations of large sets of numbers. Previously, the app did the calculations on the CPU which was a bit slower.
Borderlands works great on ipad 2 and up, so, you can really buy whatever ipad model you'd like. I'd recommend the iPad air 2, however - buying the most recent version is a good bet for making sure your hardware will still be relevant and useful in 3-5 years.
Thanks again!
Chris