I haven't played with the 24-70, but the new HD coatings are really nice for low-effort editing, which is probably the kind of editing most of us want to undertake during the holidays. So, I'll vote for the 24-70. The 31mm is really special, but I've been eyeing the 24-70, despite owning it.
I don't think the extra light from the Sigma makes sense on full frame, in exchange for the weight trade-off. For reasonably close portraits, wouldn't the depth of field be too shallow on 35mm?
If you don't use 17-50 on APS-C, maybe you should look at the D FA* 70-200? I'm loving mine for portraiture.
Last edited by lithedreamer; 11-22-2016 at 01:30 PM.