If you can afford it - go for as much memory as you can.
Certainly I agree with the poster that says 16gb is hard to fill with music and books it isn't impssible.
For a youngster, who will hopefully get some years of life out of it - not so much.
Youngsters will certainly be swapping videos, and music etc and 16gb can get filled quite quickly in that case.
Plus don't forget - 16gb is the total memory. nearly 3gb is taken with the operating system.
For comparrison 16gb on my iPhone is taken up with 1144 songs, 7 videos, 688 photos and 120 apps - some of which do take a lot of space. Coastal navigation charts and TomTom eat memory (about 2gb between them)