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

Not so. At the same focus distance, wide open, the 17-55mm has shallower depth of field. This does not mean it has better bokeh (background blur). The 24-105L has better bokeh than the 17-55mm, regardless of the aperture used.

Furthermore, with extra focal length comes more control over DOF. At 105mm f/4, DOF is almost the same as 55mm f/2.8 - it's less than 2cm difference. And even if DOF was identical, the 24-105L will still produce better bokeh.