The Bytec enclosures from Newegg are very fast. If you went the do it yourself route, you have the option to possibly remove the hard drive and add a cdrw, dvd burner, or something else to the enclosure if your needs change. You may not be able to do that with a pre-packaged unit.
It also may be easier to change the size of hard drive in the enclosure if you buy it separate. I'm not sure how easy it is to crack open something like a Maxtor enclosure. Never owned one, and thus never tried...
If you can get a nice deal on a normal hd, then I say do it yourself.