I'm an IT guy who flies GA. I have an iPhone4s with 64GB. Here's the breakdown of storage:
Audio 4.5GB with 530 songs
Photos 175 MB with 100 photos
Video 0MB for 0 Vids
Apps 500 MB for 20 Apps (only 4 aviation apps: DTC DUAT, iDispatch, METARspk, and AOPA's FLYq)
Other (OS?) 800 MB
Leaving me with 51GB free. I could squeeze and get by with the 16G model, would feel comfortable with 32, and have 64 cause it's an i-Hand-Me-Down from the wife (also IT) and she has to have the bleeding edge (excepting husbands).
I primarily use a PC for all aviation related stuff (wx, planning routes, printing plates, etc), the iPhones apps allow me to monitor.
Hope that helps.