Between those two I would suggest the 16-35. On your 40D, the 24mm is not very wide: it's close to a "normal" angle of view. Can you really live without ever shooting wide or ultra wide? I couldn't. You have a 50mm, which is a nice short tele on your 40D, and I personally wouldn't miss the gap between 35mm and 50mm.


That said, I would strongly urge you to reconsider your "full frame only" restriction. I think it only makes sense if you are diametrically opposed to selling any lenses (even at a loss of only around 20%), you're certain that you cannot keep a 1.6X body after you upgrade, and you don't mind paying over twice as much for lower image quality in the mean time.


I also suggest reading this thread for information about why you might want to *stay* with 1.6X:


Advantages of full frame?