External Harddrive, Prefab or Construct my own

Alright so I've been thinking should I just buy a preconstructed harddrive from WD like one of the Pro Book series or try and find an internal drive and just get an enclosure. It seems to be cheaper doing it by the latter. Not to mention I can get a case that looks pretty sweet and has leds.

some will tell you it is cheaper to build your own others will tell you it is cheaper to get a prefab. when i look around it looks like the major chains (best buy, circuit city, compusa) will offer a sale on a prefab hard drive at a price that is very hard to beat when building your own. the advantage of building your own is you can always up grade the size of your hard drive.