I'd recommend a Firewire drive, my Mac never likes mounting USB drives. Also, don't buy a pre-made one, for less money you can buy a caddy (I like the Icy Box) and a larger drive (I like Samsung, v. quiet). IDE vs SATA probably isn't so important in an external drive, as they're already not super-fast.
Remember you're not going to be able to use the same one for both computers as it comes, the standard Windows formatting is NTFS, and Macs use HFS+. If you want to use it on both, you can buy software called MacDrive to let Windows read HFS drives.