Actually, you forgot one other option. You could buy a regular 3.5" hard drive and an external USB case too. All things being equal (brand and model of hard drive or at least - grade of hard drive), there's really no technical difference between buying a pre-built external drive and building one yourself with a hard drive and an external case. With the drive and case, you get to put it together. You should choose what you want based on price and what you get for your money. For example, the pre-built might be bundled with backup software for free. But it may not have the Firewire connection you want. So it's all about choice and value for your dollar. An advantage to buying the build it yourself option is that you get to choose the hard drive size and you get to replace the drive if you need a different one. Pre-built external drives are not always upgradeable. Regarding 2.5" versus 3.5", there are some people that believe the 3.5" drive is more robust than the 2.5" due to its ability to shed heat more efficiently. So if I were choosing based on reliability and ruggedness, I'd probably go with the 3.5" drive. The 2.5" drive has one advantage though - it can often connect to and get all of its power right from the USB port - without the need for a separate power adaptor. If this drive is going to be very mobile, having the smaller one with no power adaptor is a real bonus.