[scollins]

I'm a Nikon shooter, so things don't exactly match up, but....

My lenses on a D300 (1.5x crop) are
17-35 f2.8
50mm f1.8
70-200 VR f2.8

I love the 17-35, especially since it has a ridiculous close focus ability (11", from the back!) I'm sure the Canon delivers similar image results.

But, that zoom gap from 35mm to 70mm has caused me some problems in certain situations. I was shooting my daughter's pre-school graduation, and the 24-70 was the lens I needed. The 17-35 was too short, the 70-200 was too long, and the 50mm wasn't versatile enough. I couldn't move forward/back to frame shots, since every other parent was trying to get shots too.

So I'm probably going to add the 24-70 at some point, and I may even sell the 17-35 and go with the 14-24, especially if I step up to a D700 or other Nikon full-frame. But the 17-35 has the advantage in being able to take filters that the 14-24 does not.

Anyway, not that much of that does you any good for deciding on Canon lenses. So it comes down to what you shoot the most I suppose. If the 17-40 is your main "go to" lens, and you don't feel like you are coming up short, then go for the 16-35. But if you are always thinking "if I could just get a few more mm's out of this thing", then certainly choose the 24-70mm.

Better yet, keep the 17-40, sell the 100mm Macro and get the 24-70! Don't know if the budget supports that though...