Assuming you mean to buy them for your car player and not your laptop...
As it says in the Drive Upgrade Guide, get the cheaper one. The speed of the faster and more expensive one will be wasted on the car player.
Exact quote:
Disk drive access speed isn't a factor, since the player uses an aggressive caching algorithm that leaves the disks spun down most of the time. Any modern laptop hard disk should be fast enough, and you will not notice any speed increase by purchasing a faster disk drive. In fact, if choosing between a fast drive and a slower drive, the slower drive is probably the better choice because it will most likely be cheaper and produce less heat. Your biggest concern will be the price/capacity trade-off.