Chris, you might visit http://members.sailnet.com/messageboard/
and look for the 'Documentation' thread in the Cruising Section.
As you already seem to appreciate, there is no straightforward answer to your question insofar as I'm aware. What nationality you choose will shape your equipment
and fit-out obligations (how capable is the French yard of e.g. fitting the boat out in compliance with Oz or Canadian requirements?), where you plan to spend most of your cruising time (there would be some advantage for an EU-registered boat if/when cruising in EU waters), how the various tax obligations stack up against one another (if registering in the EU, don't forget to compare VAT rates as they vary substantially from one member
state to the next; would you even - ever - pay a Canadian tax unless subsequently cruising long-term in Canadian waters?), and where you "think" you will ultimately resell the boat (which might require a buyer e.g. in Oz to reregister and be obligated to new tax liabilities, and which would inturn unavoidably lower your sales price
when he compared your boat to other 'domestic' offerings).
Some feel that choosing a flag that's maximally 'acceptable' within the cruising venues they are planning is a good idea. (U.S. is "bad", Britain's Red Duster is "good") but for the cruising destinations you plan, I doubt that will make a big difference given your choices.
I can't imagine actually wanting to stay encumbered with French bureaucracy long-term, but that's a personal reaction of mine which you may not share. OTOH I am unaware of any French equipment
requirements that you wouldn't otherwise want to honor given your offshore
sailling plans - what do you think they might be?
All of that being said, if you really do plan to resell in Oz some day, I'm guessing 0z registration will ultimately prove a good choice when you look at the above variables.