Assuming all your devices have clean heads, clean connectors, good cables & properly working electronics and transports, it's a toss up. It would be best if you could try both methods and see for yourself which gives better results, if there is any difference at all.
Your Hi8 camera probably only has a Composite video out, which is not the best quality signal, but using the
ADVC300 may correct for that and more because it has TBC correction and some other video cleaning & enhancement capabilities that a deck or camcorder alone won't have.
Using a deck that can play the 8mm analog tape and convert directly to DV over Firewire avoids the degradation from a composite analog signal, but you won't have the TBC and cleaning capabilities that the Canopus would give you.