Which among the Canon EF 16-35 F2.8L MKII or the Canon EF-S 17-55 F2.8 IS lenses shall give better image quality using the Canon EOS 40D camera with a crop factor of 1.6?And why?
Thanks.

Your "head to head" may not be legitimate...they fall into different "end user" brackets

1. The 16-35 is "L" glass and the 17-55 is 'Consumer Grade"
2. The 17-55 is "IS" equipped and the 16-35 is not
3. Canon lists the 16-35 as a "wide angle zoom" and the 17-55 as a "standard zoom"

However, these and other factors notwithstanding, in my humble opinion the 16-35 will easily produce "better image quality"...

This assumes one compares them using identical aperatures and focal lengths, something they share over a somewhat limited range

Whether one or the other is the lens you should consider, or will meet your requirements/needs was a question not posed in this topic.

Both will provide excellent image quality. I don't think you can choose between them based on image quality. However, the L lens will likely last much longer and can be used on any EOS camera, not just those with an EF-S mount. The IS mechanism on the 17-55 breaks too easily. Mine broke after a few months and was repaired under warranty.

17-55 IS.

More range. Tests on a crop camera have it and the 16-35 very close. (Check photozone.de) Has IS. What's not to love?

Tests over at www.slrgear.com seem to have the 17-55 quite a lot sharper.

And it has more range and IS.

But it is EF-S v L with all those trade-offs.