If transport isnt an issue get a 3.5" one (you wont get a 2.5" one above 120GB anyway). Bare in mind 3.5" drives also require a power supply (supplied) where as 2.5" drives will generally get their power from the USB port.

Consider buying a case from ebay (about £15) and a hard drive of the size you want seperately and fitting it your self - its very easy to do and that way you get exactly what you want.