It sounds like a very good problem to me.

Given that you have the 50D at home, I can understand that it wouldn't be easy to part with the 17-55. If you have the patience I'd suggest starting out with the 28-135. It would give you the chance to cover the focal length ranges of the 24-70 and the 24-105 to get a better idea of which might be better suited to your needs. Landscapes can be done well with either, while f4 for portraits might be too narrow and 70mm might come up short depending on your preferences. The time spent evaluating just might give you the opportunity to save enough for a secondhand copy of either. Once the 24-70 II comes out, I'd think that there might be enough of the initial version being sold off to drive the price down a little bit.