Canon 28mm 1.8 vs 24-105l IS

I think there is a good cause to have a fast normal prime in addition to the 24-105.  I wouldn't get any of the 2.8 IS ones though because on crop you would be better getting the Sigma 17-50 and still have stabilization, still be sharp, and still have 2.8, but you can zoom for the same or less cost.

I personally went with the Sigma 30/1.4, but any of the following would be money well spent:

28/1.8, 35/2, 35/2 IS, Sigma 30/1.4, or Sigma 35/1.4.