It's my opinion, but the external cases use their own/buy the cheapest controlers for the USB/Firewire bridge for the harddrives. There are a very few that use good quality components that I have seen. From the external drives that I have used, I would say that seagate is the best hands down. Maxtor is the worst. Lost 3 of 5 controlers for the cases and had to move the drives internal.
Now with SATA going external like SCSI, I don't know what to tell you as I have not had the chance to plat with them... But I should in the next few months..
Hope this helps.