What's the best way to share an Aperture library between two users on one Mac?

Should I go with an external Firewire 400 drive (sadly, I don’t have a Firewire 800 cxn on this particular Mac), USB 2.0, or a shared folder? Other suggestions are welcome.

I should note that I not only want both users to be able to view the photos, but also be able add, delete and modify photos from the shared source.
 
indifferent I’m wishing there was an easier way to do this so I wouldn't have to ask the question in the first place.
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