a few things to consider:
Firewire only - cheaper
USB 2.0 only - cheaper
Combo (USB 2.0 + FireWire) - more expensive
3.5 inch (Hard drives only) - cheaper
5.25 inch (Hard drive OR optical drive) - a bit more expensive
*a 5.25" case may allow for better airflow & cooling if it has a built in fan. Aluminum cases will also be a bit more sturdy and cool better than plastic, but cost more.
Oxford 911 (firewire only - recommended)
Prolific (I have a case with the PL3507 USB/firewire combo chip & a 160GB WD SE drive in it, connected via firewire. I have experienced a few "write cache failed and something about $mft data lost." type errors when writing to the drive (usually larger files +100MB). Basically the file being written at the time becomes corrupt, and the drive disappears from windows untill I uplug it, reboot and reconnect it. ScanDisk does not find any errors or bad sectors (one time, I even installed the drive via IDE inside my computer to scan it "just to make sure".)
This is not a problem, since the drive is just backup/temporary file storage, and has only happend two or three times in the last 6 months.
//* slightly off topic
I have achieved transfers up to about 32MB/sec via the firewire connection, though usually file copies are around 20MB/sec (at least based on the program I use - Total Copy. It is old, but works well. It is very hard to find online, since it is so old, but I am not sure if I can/should post it. It was freeware, so I do not think it is a problem. It even allows copy transfers to be "paused" as well as setting a limit of the transfer speed. Perhaps a mod could comment.