You don't say whether you're using Tiger or Leopard. Presumably if you're using SuperDuper! and are aware that it's not Leopard-compatible, you're still on Tiger. I've been warned away from using a WD My Book with Leopard, so if you're planning to upgrade in the near future, that might be something to keep in mind. My recommendation would probably be to get a regular (internal) Seagate drive + a multi-interface enclosure from OWC. I haven't found an external firewire drive I really, really like.
Some more opinions:
http://shirt-pocket.com/forums/showthread.php?t=3091
Good luck!