I've had the 24-70 for a while, just added the 24-105 because there are frequently times I need that little more reach and the 70-200 doesn't focus close enough (indoors, doing pets). However, having tried it, I wouldn't recommend it unless you have a requirement for something the 24-105 uniquely brings to the table (IS or the closer focus than your 70-200). I see more Cos^4 light falloff, the hood's a wimpy, ineffective little thing, and the zoom and focus rings on the 24-105 just seem to be positioned less well than they are on the 24-70 (part of that may be the barrel diameter); at any rate I found myself jostling the focussing ring when trying to zoom more often than I like. I don't use a grip, so I don't see that as important either. Battery life's quite good without. Second flash, yes - get one.