Although the difference in price between externals and internals ain't what they used to be, I wouldn't personally buy an external. Last I checked most if not all external drives have only 1 year warranties which is another reason not to go that route.
And 1 year is far from the shortest warranty length that WD has ever offered on any hard drive. A few years ago, some of their low-end internal hard drives were warrantied for only 4 months (120 days) or 6 months (180 days).
And if an external hard drive is used primarily for backup storage purposes, USB 2.0 High-Speed (60 MB/s theoretical maximum; 32 MB/s practical maximum) would suffice. An eSATA interface on an external hard drive is recommended if you're also going to be using that same external hard drive to run programs (especially games) off of, not just data or video storage.