There are only a handful of hard drive manufacturers in the world (Seagate, Western Digital, Samsung, Hitachi, Toshiba, Fujitsu) and pretty much all of them make decent drives (Hitachi being the least reputable; Toshiba and Fujitsu only make 1", 1.8", and/or 2.5" drives which won't be available at 500GB for less than $100).
So you can rest assured that the actual hard drive, regardless of the brand, will be of decent quality.
The major differences are going to lie in the quality of the chassis, the quality of the cooling, the quality of the interface controller(s), the pre-loaded software, the interface (USB 2.0, Firewire, eSATA, and/or Network), and warranty.
My personal preference is to pick up a nice aluminum chassis ($50-$75) with USB 2.0 and Firewire or eSATA and pick up an OEM SATA drive from Seagate, WD, or Samsung.