It's the data transfer speed, the higher the better. Most are ATA133 now for ATA and SATA150 for SATA.
Pro's and Con's to both. If you wanted to protect your media from everyone, you could use your external as the default drive for your applications and remove it from your PC whenever you desired and even place it in a fire safe for further protection. Would I go that far? No, since I use my external’s as FULL backup's anyway. So after all these posts, when are you going to make the decision to build your own external drive?
I just want some information before I buy one. Im not sure if im going to buy one know or when I get my other laptop (over the summer) but I will be getting one.