Well I like to store HD copies of tv shows on my device and can store about 3 episodes of tv shows right now on the 16 GB only.
So I think 128 GB would help a lot.
How much actual space do you get at 128 GB ?
My iPhone 6 (iOS 9.2) has 114GB and my 6S Plus (iOS 9.2.1) 113GB. Should be about the same for the iPad mini 4.