I'll tell you right now that my G0 1.2625 vs the 1.3000, its very easy to see that the lower vid one OCs better. I suppose it could be just coincidence. Thats always a possibility.
But it seems to fit the voltage patterns that I see pretty well. I've noticed that you reach a point on the q6600 where it simply needs alot more voltage than speed gains. For instance, at about 3.6, the voltage needs of my 1.2625 processor hits that area where it takes alot to reach higher speeds. (3.6 is 1.38 loaded to be stable, and at 3.8 it needs 1.489 or so Loaded to be stable at 3.8.
So its great up to 3.6, but tough on it up farther. The 1.3000 one seems to reach this area sooner. And to even get 3.6 it needs 1.43 or a lil more to be stable, only at 3 hours so far on small ftts. Though up till near 3.4 you get great voltage to speed gains. After wards it needs more voltage to get the higher speeds.
I suppose it may not be the VID at all. But if I had the same processor as you, and started at 1.2500 vs 1.3000 thats like 10 voltage notches you dont have that I would! And at 1.3000 my lower VID one would already be stable at 3.0 or a bit higher.
Now since they are all rated at 1.5 volts max, to reach the max voltage while OCing, the higher VID gets there way sooner, therefor limiting the OC. (Yes, the processors can at least take 1.78 in windows, dont ask how I know, and they can take 100c before turning off.)
You can always go over the safety limits of what most others do, if you want to, and reach higher speeds. But as far as the better processor goes, I would hand it to the lower VID. Lower VID = less voltage and heat at whatever speed.
Then again, I have only got what others say, and what they report about their OC, and the VID of their quad. I personally have only tested 3. I will tell you what, the 1.3125 VID B3 I had was friggin' hot at any OC, lol!
--Lupi