Ignore the CPU that's inside the camera, period. Putting a Digic99 into a camera instead of a Digic.001 just means that there's more computational power to execute the task of building the image files. If there was a way to remove the Digic5 from a 70D and drop in a Digic3, I'd bet you'd see a significant frame rate decrease. Likewise, if a CPU upgrade was possible, you'd probably see a more consistent frame rate at high ISO speeds.

The 1-series is going to have a higher quality sensor and a higher-capability A/D converter on the back end of the sensor. However, generational improvements have led to better sensor manufacturing techniques that provide a better image quality. If you're truly talking about a 1D2, you're talking about 2004 technology, which would be second-generation 1D vintage. The 550D is 2010 vintage, while the 70D is 2013 vintage. I would simply expect better image quality every two years, so the 70D is probably the winner there.