Re: Canon 17-40 f4 or a 24-105 f4
Something wrong with the Canon 17-85mm? I'd have to have a very good reason not to buy that(or maybe a Sigma 17-70)....it works fine as a walkabout lens on APS-C IMHO.
The 17-40 and 24-105 make more sense to me when used on a full frame body - on APS-C the 17-85mm is pretty much as good as having both of them without having to change lenses and I'd suggest that as you already have a 70-200 (with IS?) that the long end of the 24-104 is not of much benefit. With the 17-40 you wouldn't get IS which could prove useful on a walkabout lens.
Andrew
A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away. - Antoine de Saint-Exupéry