I wouldn't worry too much about the speed difference between ports. Opening a RAW file of few tens of megs takes a few seconds, and even doubling that time is not an issue for most people. Your main HDD or a separate scratch drive(s) are the ones that bottleneck your PS performance - not the drive you store your RAW files at since you access them only twice (once reading, once writing).
Compatibility is more important if you plan on moving the drive between different computers and/or systems (Mac vs PC). USB 2.0 wins hands down on that.
As for drives, I've used a Hyperdrive SPACE for half a year, and love it. It works as an external HDD wonderfully. I use it to transfer my RAWs from memory cards in the field, and as an off-computer second backup drive. If you choose to get one, get the casing-only and buy and install your own store-bought HDD to save bucks.
There are several other portable 2.5" HDD casings out there, but they are generally realllllly slow as external HDDs due to design issues - slow enough to make a difference. HDDs marketed as "external" rather than "portable" are of the fast type. Usually. Yes, it's a pain, read reviews, verify transfer speeds, caveat emptor, etc.