If the 24-105 creeps and extends by itself try a rubber band around the lens barrel.after a month I admit I spend more time flopping shutters out of the 17-40 than the 24-105 now. One thing I have to appreciate is not having a lens extend open while I hike somewhere (I use a peak cam clip on my backpack) and the difference in pics at same mm is just that. The same.
Still planing a long lens again and now I think it will be sigma 150-600 as I used one the other day and it didnt disappoint.
This damn hobby is addicting ... and expensive. Not necessarily in that order.
I looked at the sample pics and have some friends posting online who swear by them. I do think an "L" lens is hard to beat, except for the priceI don't think you'll be disappointed in the big Sigma at all. I still want a few more ART lenses but am holding back for now.