I did a qiuck comparison of the Canon 24-105mm 4.0L and Canon 24-70mm 2.8L to see how they compare. The 24-105 is mine, and the 24-70 belongs to jani on this forum. I shoot with my 5D and the result can be seen here: lens test
Jani did the same teste with his 20D and can probably tell about his findings if he ever get som spare time.
My personal conclusion is that the lenses almost have the same performance at same f-stop. The 24-70 was sligthly better in the edge at 70mm. But handheld the 24-105 would probably be better with the IS.