... the 24-105 is better, especially since the long end delivers pretty decent results as far as background blur goes - more or less a wash compared to f/2.8 at 70mm.
I think statements like this are a bit misleading. Yes, if you're at the same distance to your subject, 105mm f/4 actually gives shallower DoF. Heck, at the same subject distance, my 100-400 at 400mm and stopped down to f/22 is shallower than the 85L at f/1.2. But it's not the same picture at all. A head/torso shot at 70mm becomes a tight head shot at 105mm, and as soon as you back up to match the framing at 70mm, you've negated the effect of the longer focal length on DoF with the opposing effect of greater subject distance. So, for the same framing (with the same sensor size), it all comes down to aperture, and f/2.8 is wider than f/4.
Which means if you want shallow DoF, a fast prime will beat a zoom. Since the OP already has the fast primes, and wants versatility, I think (personally) that the 24-105 is better for that.