In terms of the hardware, IMHO there's not all that much difference between the drives in terms of reliability or speed. What's probably more important is to choose a drive that has the features you expect to need. Features could include:
- choice of interfaces. USB 2.0 is fairly standard but slow - eSATA will be much faster if your computer has an eSATA port and you don't mind the relatively short cables that you have to use with eSATA. USB 3.0 is just starting to become available and for external drives will be similar to eSATA in performance. You may want to buy a drive that supports both eSATA and USB for maximum flexibility in the future.
- packaged software. If you don't already have a backup package then some drives come with backup software included.
- bells and whistles such as a button to automatically start backups or folder synchronization or a built-in USB hub. I personally prefer just a simple, no-frills drive but you may find a use for features such as this.