I have work experience with the 5D2, 5D3 (which I own now) and the 7D:
The 5D3 uses binning instead of line skipping to downsize the sensor resolution to HD, which reduces moiré and aliasing by an incredible amount. There is some aliasing in overblown highlights which is a problem that all single large frame sensor cameras share (including the Arri Alexa), but there are ways to get rid of that in post.
Except for a lack of resolution in comparison to the C100 / C300, which also operate in an 8bit color space, the 5D3 is a capable video camera, probably the first DSLRs really made for it. Apart from that the audio circuitry is also much better than the 5D2 (if that would matter). What might improve the resolution a little bit is to tweak the h264.ini settings with ML. Mine is stable (also on complex scenes) recording at an inter-frame data rate between 69 - 120 mbit/s, and there is a definitive increase in detail, especially when sharpening is applied in post. In addition, you could also use a cheap external HDMi recorder (like the soon-to-be-available Ninja Star by Atomos) to record from the "4:2:2 uncompressed" HDMi out, but apart from getting rid of the h.264 blocking, it doesn't really give you anything "more" color- or resolution-wise.
When it comes to ML raw recording is where the 5D3 really shines (again, more detail and of course high color depth). Recording at (true) 1080-25p is possible with the right CF cards. In total, the image quality blows away the C300 and the 1D-C. But that comes at the price of a more demanding / time consuming post workflow.
In my opinion, an upgrade to a well-tweaked 5D3 is way worth the cost.