That's the problem with digital bodies nowadays, your reliance on lens increases and that's where Canon get their advantage from.
Back in the early 90s where Nikon, Canon and Minolta and fighting it out where Nikon already gets into most of the photographer dry cabi, Canon caught up with their better technology glasses aimed with a good timing towards the increased opportunity for faster lens.
Nowadays, with the crop factor thing keep bugging many Canon lenses users, you will be disturbed that many "used to be" excellent lenses such as the 70-200, 28-70, 17-40 becomes awkwardly unusable with the funny range. Usually many prosumer users that have L lenses in the past had to stick with the less attractive digital lenses because Canon can't produce them fast and at professional quality. Do bear in mind that utimately, if you get the digital lenses, in no time, newer and full frame DSLR will flood the market and eventually makes them less attractive to be used. Unlike many of the lenses that I've got 10-20 years ago, they still work a charm on the latest DSLR.
Coming back to your lens question, because of the focal length, weight, crop factor blar blar blar, I think the best solution would be of course to weight your concerns first. Since you have the 28-135, I believe its good enough, for a full frame body of course.
If you are concern with landscape angles, then of course you need 17-40. With a digital output, you can in fact crop in to get details, but that is of course at the expense of manipulating original framing which most digital users are not too concern with. If you are good at your post photography developing, and after cropping in, the output is still acceptable with the resolution your camera bady produced, then getting a 17-40 is a better choice.
I've both lenses and have experience in carrying only one of them out to shoot. I've use the 28-135 on a 11 day trip to Beijing and captured almost 100% landscape photos. I've also used only a 17-40 locally for outing shoots and nothing else. I get satisfactory shots on both occasions.
I believe it depends on the way you frame your subjects and how you want to present the captured photo, either lens is in fact, a good choice because whatever you planned on taking may eventually comes out to not the case. My recent trip to Jiu Zhai Gou in Chengdu, China eventually turns out that I use my 300mm more to capture wildlife and details as opposed to the belief that wide angle will be used most of the time.