This is an easy one. If you can afford to, buy the 5D Mark III and keep the 40D as backup. You'll have a great FF and a good 1.6x crop sensor camera. If I were purchasing new and had the glass I needed, I'd probably buy a 5D Mk III and a 60D backup body, if my budget was $4300-$4500. But since you ALREADY HAVE the 40D, keep it and get a 5D Mk III.