32 is what i've done on all iOS devices for all family members for years, its worked flawlessly in keeping anyone from having to micromanage storage over the life of the device.
think of it this way, theres a baseline amount of software both iOS and the apps standard apps you will want to have, your actual usable free space will effectively be after that is subtracted.
a 16 gb realistically will only have maybe half of the 16 available for most people. the next bump up actually does more than double the effective space you will have available.
that said, i had 64 gb once and it was a retarded waste of money that never was utilized, maybe if someone kept a large media library on the device 64+ would have a use.