although at the moment I already have quite a few things on the wish list to come, not to far away is a new everyday zoom lens.
I want to make an upgrade from the 18-55 kit lens.

it would be used for Food Photography alot, as well as holidays, (family travels a fair bit, Vietnam in mid July next year) and also for social occasions such as birthdays at homes and restaurants.

I will be using it on the Canon 60D (which is a 1.6x crop)

I've been looking at two lenses most closely, that being the 17-40 F4 USM L lens and the 24-105 F4 USM Lens.

From what I've learnt and heard the L lenses are at the top of the range when it comes to Canon lenses, made obvious thanks to the large price tags.
But is it there much point for me to be looking at the L lenses when my Body isn't weather sealed and there are lenses such as the 15-85 F3.5-5.6 IS USM lenses available, or other similar zoom lenses from Tamron and Sigma?
The 15-85 offers a wider zoom range, slightly faster, has the addition of IS and is a little bit cheaper. However it obviously isn't an L lens, I guess what my question is ultimately leading to is what other advantages do the L lenses posses? Is there a noticeable difference in IQ, or focusing speed etc. ?

Lastly I know that the 15-85 is an ef-s Lens, but I don't see an upgrade to a full frame camera for a long if ever.