Personally go for the 24-70L. You obviously have full frame in mind so go for it. I would never pay the dollars for a 17-55. Yes it has IS but really at that focul length its just a plus. IS is great for long telephoto zooms ie 100+. But anything lower than 100 doesnt really need to have IS built into it (plus you will have a 7D and you can increase the iso if you need to shoot in lower light). Justifing the 1000 price tag for the 17-55 is tough because its so close to the price of quality built L lenses. Yes he could sell the 17-55 but hes not going to recoup all 1000 back. Do yourself a favor and buy the 28-70L or the 17-40L. You will get great optics plus a durably lense which you can take out there and not worry that it isnt as tough as the body you have. You will save yourself money in the long run and the hassle of having to sell off lenses.
While I understand your point about L quality glass/IS and the price of the 17-55 etc, I'm not really sure the 24-70 is the best solution for the OP as he/she wants the lens for landscape photography. 24mm on a crop sensor isn't all that wide and may be quite restrictive. Obviously its all down to the individual and their photography style, but something to consider.




