X-bit Labs has a review of nine external 2.5" hard drives:
Mobile storage devices are still in our focus and this review is about 160GB models based on 2.5” hard disk drives. It is not a record-breaking capacity as there are 250GB (for details see our article called Roundup: Five 2.5-Inch Hard Disk Drives with 250GB Storage Capacity) and 300GB models available, with two and three platters, respectively, and Western Digital has recently announced a WD Passport with a capacity of 320GB.
160GB external storage devices are popular today. Such drives are considerably cheaper than those with larger capacities, and they are superior to 80GB and 120GB models in terms of price/capacity ratio.