All the discussion in this forum about the 5D3 in low light has me intrigued. I'm particularly interested in a 5D3 w/24-105 f4L IS as an upgrade path for a 7D w/17-55 f2.8 IS. (For low light candids, I'm often shooting at 1/30 second and find IS to be a must.)
I know that the 5D3 offers greater color depth and that full frame is typically sharper than crop. I also understand that the 5D3 offers about a 1.3 stop advantage in noise over the 7D. But, I'm comparing a crop body with a 2.8 lens to the full frame with a 4.0 lens and this noise advantage drops to about 1/3 of a stop.
Now the question. A 2.8 lens lets in more light than a 4.0, which is more light for the AF system to lock in. So which system can lock in on focus at lower light -- the 5D3 w/24-105 f4L IS or the 7D w/17-55 f2.8L IS?
You might also consider a 6D, with a 24-70 f/2.8 lens. Your cost would be similar, if not less, than a 5D3/24-105 kit (depending on which 24-70 you choose).
I feel the 6D is superior to the 5D3 for low light. I'm not alone. The 5D3 is best for slightly more than low light, and with very fast, erratic subjects. You won't be achieving your goal with the 24-105 lens. It's a great lens, but not in low light. It also doesn't AF quickly even at noon on a sunny day,
no matter what body it's on. I personally would buy a 6D and the Tamron 24-70.
As for the crop factor...I also upgraded from a crop camera (50D). I've only had my 6D a week, but have shot over 1000 pictures, still haven't tried all of my lenses. All I can say is, what the full frame fanboys have been saying is true: A cropped image done with either the 6D or 5D3, will be more detailed than you think it will. It will not be as detailed as a crop body in good light, but it won't be 1.6x behind. It will be about 1.25x to 1.35x behind in good light, and about 1.1x behind in light requiring up to ISO 2000 or so. Above that, it will be ahead.
These differences have become negligible from a practical standpoint, in my opinion. I no longer see a need for a crop camera (and I never thought I would not. I will be very sad to see mine go).
This evening I shot an image at ISO 8000 with my Voigtlander 58mm f/1.4, in dim outdoor light, with all in-camera NR turned off. I had it closed to about f/10. The detail is beyond anything I have ever seen short of a D800 paired with whatever their choice of sharpest lens could be, but at ISO's below 1600 for the D800. The luminance noise in the shot I'm describing, is as low as my 50D at perhaps ISO 800 or 640. The chrominance noise is similar. It's there, but with very slight NR in post, it's gone, and all the detail remains. The color depth is surprisingly nice, though the lens makes a big difference. You need a lens that is fantastic at color rendition, to make full use of the 6D.
I'm blown away by this. Yes, the physical size of the 6D, makes it seem like a toy worthy of derision by the 5D3 or 1DX crowd. It's not a toy. What it is, is what 5D2 buyers five years ago, wish they could have bought instead...and for 40% less cost to boot!