Canon 17-55mm f2.8 or a 24-105 F4 L

If you want to be very technical, and take 85mm on full frame as the ideal portrait focal length, then you need a lens that can do 53mm on crop. So both fit the bill. That leaves aperture and bokeh.

Both lenses are capable of producing pleasing bokeh, although the 24-105L is slightly better in that regard. The 17-55mm on the other hand, can produce shallower depth of field. So that's what your choice comes down to.

If you're not concerned with the popular focal lenghts, then it really comes down to your preference. Both lenses are sharp, both produce good colour. the bokeh difference is much of a muchness if you use the longer end of the 24-105L. The 17-55mm is L quality, optically.

The biggest difference between the two lenses is in build quality. The 17-55m is one of the better built consumer lenses (although it has a reputation for sucking in dust, I haven't experienced this), but it's not weather or dust sealed like the 24-105L.