I'm Ann, from the team here at Shopify :)
Great question! Something to note when deciding between the two, is how much each iPad can store. Taking a look at this guide we have in our Shopify Manual, it will show you what each iPad can hold when it comes to product numbers and variants. Although the iPad Minis aren't listed, they would fall between the iPad 2 and the iPad Air in terms of specs.
Something else to keep in mind is that the Shopify POS app works with any Apple iOS device that's running iOS 9.0 or higher, so make sure one of your models has that.
Finally, it's good to note that iPad Minis are not always as spec-ed out as standard iPads or iPad pros but they are a great option for in-person sales!
I hope this helps! If you have any more questions or concerns feel free to reach out to us here :)
Have a great one,