I'm an IT guy and I use an iPhone 4S 32GB & an iPad3 32GB. I have quite a few pictures & videos stored but not much music. For flying I use ForeFlight which IMO is the best flying app I've ever seen. It was $150/year and came with full Geo Ref approach plates/terminal procedures & taxi diagrams (moving map gps overlay). Another great feature anytime your on the ground it speaks and flashes a warning as you approach a runway. Then another alert once you actually enter the runway.
On my iPad I've downloaded all the the VFR sectionals, low altitude IFR enroute charts, approach plates/terminal procedures, and AFD/airport info for about 40 states.
On the iPad I've used around 16GB of my 32. Took around 10GB for all my ForeFlight charts etc that I downloaded...
So.. If you can afford the 64, go for it.. I'm happy with my 32. But don't get the 16.