Why not take a look at the survey given here
www.photozone.de/active/survey/querylens.jsp
You will see there is not much difference, if any, but since the 24/105 has IS I went for that.
You may well be attracted by the f2.8, but don't forget the extra weight.
Here is another useful site
http://www.luminous-landscape.com/tu...ding-mtf.shtml