My 2 cents worth:
- Harness first:
When the wind is strong enough to get you on the plane without needing to pump - Harness last:
When you have to pump to get planning, it's often easier to get into the straps when up and planning as getting into the harness may effect the trim of the board
When you're way overpowered
When learning from scratch there's no point getting to grips with the footstraps only to get knackered after 20 minutes of planning without the harness! Learn the harness and as you speed up do the "elvis shuffle" (see a Guy Cribb article) down the board. You should be able to do both pretty much in the same month...
Good luck!
BTW well rigged sails and properly placed harness lines are as important as any technique advice so make sure you get this right. Try the elastic technique that Cribb talks about on his cribb sheets - it seems to work! Where do you sail as I'm sure people will be keen to help out with rigging/line positioning?