Drive itself is important. However enclosure might be the real issue here.
Enclosures have features that can make it day or night, e.g.
1. activity light for data access just like a desktop HDD light
2. fan or good metal contact with the hard drive to keep the drive cool
3. Strongly built, e.g. aluminium cases might feel more stiff than those flimsy plastic cases
4. metal case, which helps in physical strength and might help reduce temperature of the hard drive
5. noise insulation
6. vibration damping: good cases have rubber feets sitting between the drive and the enclosure
7. Is a universal power supply included???
8. Is the enclosure bootable? (...I really don't have much knowledge on this. However not all USB memory keys are bootable, so I guess it could be the same for USB enclosures)
9. This is more related to the drive itself: will the drive being able to run on only USB power, or external power supply has to be plugged in? Some people said 2.5" drives are small enough to be run by only USB power.
However I don't know any recent models of enclosure. The only suggestion I have is to look around on sites like http://www.newegg.com
and see if any USB enclosure fits the points I listed above.
Oh another suggestion: do NOT rush when you're shopping, unless you need it like right now. I jumped the gun on Powersleeve14 laptop case, and now I kept wondering if Waterfield sleeve case would be a better choice or not.