They are different devices, period. Since Apple has never released an iPod Touch with 128GB, I decided to get the rMini to replace my aging iPod Classic. It holds my music collection (most of it, anyway; even 128GB isn't enough and music in the cloud doesn't work for me). It has very few apps, just basics for reading, podcasts, and Netflix. It's small enough to be mobile and goes with me everywhere. It's WiFi only; if I need internet, I can tether it to my phone. At age 60+, I find the screen too small for extended reading with any app other than Kindle.
My iPad Air is more general purpose use, for reading, web surfing, video, app development, reference, and more. It goes to work with me, along with my MPB and Surface Pro, but mostly I use it at home.