More info stored for buck: External Hard Drive
Faster search time:iPod or other external media player.
Understand that through the USB, I do not believe you can use the ipods menu on the deck itself. I have no experience though.
As for external drives, make sure to be able to find a power source for a hard disk drive. I'd suggest a cord that has the power supply the disk needs, and can plug into the cigarrette lighter.
A note to consider is the limitations of the current software. I believe the deck itself will only load up to 15,000 songs via external hard drive, and on average that has been tested to be roughly 16 gigs of music. So in essence, the only reason to get a 120 GB HD or more, is for future developement in deck technology allowing for loading entire hard drives. Personally, I only use 4 gigs with my current music, so I'd be fine.