It really depends on your usage and what size of the data you are backing up and how large your snapshots are.
If you use applications like entourage that create giant single db files then you might want the faster drive. Since it will need to backup more ever hour. The same thing applies to any media application dealing with large files, over 300MB or so.
For most users, firewire 400 is fine. The initial backup will be much slower, but after that its ok.
Finding an enclosure with firewire 800 isn't that hard and they aren't that much more expensive. If you aren't on a budget, just got for the best you can.
If you use applications like entourage that create giant single db files then you might want the faster drive. Since it will need to backup more ever hour. The same thing applies to any media application dealing with large files, over 300MB or so.
For most users, firewire 400 is fine. The initial backup will be much slower, but after that its ok.
Finding an enclosure with firewire 800 isn't that hard and they aren't that much more expensive. If you aren't on a budget, just got for the best you can.