Both lenses are excellent optically. Don't let the fact that the 17-55mm isn't designated as an 'L' lens put you off -- this was purely a marketing decision by Canon.

Your needs are similar to my own. I tried both lenses before settling on the 17-55mm. The 24-70mm may be the ideal walkabout lens on a full-frame body, but on a 1.6x crop, the wide end simply isn't wide enough for architecture and landscape shots.

The f2.8 aperture of the 17-55mm will also give you more flexibility creatively, and will certainly come in useful if you are shooting in low light. The one stop difference between f2.8 and f4 might not sound much, but this equates to twice as much light entering the lens, which will also give you a brighter viewfinder and improved autofocus.

Both lenses are excellent optically. Don't let the fact that the 17-55mm isn't designated as an 'L' lens put you off -- this was purely a marketing decision by Canon.

Your needs are similar to my own. I tried both lenses before settling on the 17-55mm. The 24-105mm may be the ideal walkabout lens on a full-frame body, but on a 1.6x crop, the wide end simply isn't wide enough for architecture and landscape shots.

The f2.8 aperture of the 17-55mm will also give you more flexibility creatively, and will certainly come in useful if you are shooting in low light. The one stop difference between f2.8 and f4 might not sound much, but this equates to twice as much light entering the lens, which will also give you a brighter viewfinder and improved autofocus.