My experience of external USB/FW enclosures, is that your usually better off buying the cheapest one you can find. Even the good ones will die long before they really ought to. So with that in mind, don't rely on one as a backup device. If you have to, you should rely on two (mirrored RAID or making scheduled backups of one on the other).
I personally favour firewire drives over USB for use with a mac. I find the connection to be far more reliable.