I took an internal Sata HDD and got an external enclosure. It was cheaper and I got a 250 gig 7200 RPM hdd with 16mb cache for 50$ Then I got a nice enclosure for another 50
I personally am not quite sure. But with proper research and some technical knowledge, it is usually cheaper to make your own. Unless you are looking for super portability, then just buy an external that doesn't need a power cord (aka USB powered)
Just buy an internal HD and buy a case bada bing there ya go but if you want it USB powered there are other ways of doing that even with an external HD