First of all, I own a 6D with the Tamron 24-70 VC and am very happy with this combination.
True, the 5Dm3 offers many more software features and more AF points, if you are willing to use them. If you don't plan on using more AF points (I only use the center one) or if you don't like to fiddle in the menu to finetune the tracking features for birds in flight (or similar), then you'd be probably better off with the 6D.
With the big price difference in body, you might even consider to buy a low light lens like the Canon 50mm F1.4 and a flash (still in the price for the body difference).
Alternative, you buy the 6D without any additional gear (besides lens) and risk that it maybe once breaks. In this case you have saved so much money that you can buy half a 6D again with just the savings