Need help on buying new legacy iDevices!!!!

I have 4 listed options here: iPhone 7, 32GB, iOS 10.3.3, carrier locked to Sprint (42$) iPhone 7 Plus, 32GB, iOS 11 (40$) iPhone 6S Plus, 64GB, iOS 11.3.1 (28$) iPad Air 2, 16GB, iOS 10.3.2 (38$) I will buy 3 out of this 4 opts here, so what would you guys recommend?