If there's a deal on pre-fab'd external hdd is cheaper but on the whole they are more expensive then buying an interal drive + enclosure seperatly.
Consider this. External drives have 1yr warranties. If something goes wrong (which is almost always the sata/ide -> USB/firewire controller board, you can not open up the enclosure, hook up the drive internally to save your data and return the external drive for warranty purposes. However, by buying them seperatly you get the longer warranty for the hdd (3 or 5yrs), you get to choose what features you want on your enclosure (cooling, connectivity. enclosures seem to have 1 yr warranties), and you don't have to fully assemble the enclosure either (provide better cooling for the hdd). Should one or the other fail you can test to see which it is, and as it will probley be the enclosure that fails you can keep your data and get a replacement.