I currently have a Canon 40D and the main body lens is a Sigma 17-70 DC Macro. I also have a Canon 50mm f1.8 and a 70-300mm IS USM.
I have the opportunity to buy a friend's 5D Mk 1 body because he's upgrading to a 5D Mk2. That would mean I'd have to buy a new body lens (although I'd probably buy 2nd hand)- which would probably be the 17-40 L or 16-35 L.
I believe that picture quality is largely comparable between the 5D and 40D (?) so my main motive is to get wider angle shots. So should I simply go for a 10-22m EFS to use with my 40D? This will obviously be a cheaper route....but further restricts me to 1.6x crop cameras (although I could of course sell the lot in future).