See the detailed proof and picture quality comparison between FiOS and Comcast at AVSForum
Forum member BFDTV has been conducting a detailed and intense comparison of HDTV signals being broadcast by various companies, Comcast and Verizon included. His study continues and all of his findings along with the details and sources are included.
Here is one example…you be the judge:

FiOS - Red Hot Chilli Peppers Live on MHD

ComCrap - Red Hot Chili Peppers Live on MHD
There are pleanty more examples availble. For those of you that have been on Twitter or other forums complaining about pixelation and the overall Comcast signal quality…IT MAY NOT BE THE SIGNAL AT ALL!!! It may be the result of Comcast OVER COMPRESSING certain channels so that they can fit more garbage channels into their lineup.
Here is a sample of the SIZE of the signal being sent down the pipe by these two carriers. The difference in size directly translates to picture quality.
FiOS Comcast Difference
AETV HD 18.66 Mbps 14.48 Mbps -28.9%
Discovery HD 14.16 Mbps 10.43 Mbps -35.8%
Discovery HD Theater 17.45 Mbps 12.60 Mbps -38.5%
Food Network HD 14.32 Mbps 13.73 Mbps -4.3%
HGTV HD 14.76 Mbps 12.43 Mbps -18.7%
MHD 17.73 Mbps 13.21 Mbps -34.2%
National Geographic HD 13.40 Mbps 11.92 Mbps -12.4%
Universal HD 12.72 Mbps 11.01 Mbps -15.5%
HBO HD 8.87 Mbps 8.81 Mbps -0.7%
Cinemax HD 11.40 Mbps 10.77 Mbps -5.8%
Starz HD 11.93 Mbps 9.76 Mbps -22.2%
BFDTV comments, “The greatest differences are seen with movement. With slow movement on Comcast, the first thing you notice is added noise and a softer image, as fine detail is filtered from the picture signal. The greater the rate of movement, the more detail you lose and the more noise you see. With intense movement, you see more blocking and skipped frames. In VideoRedo, I noticed that a number of frames in the FiOS signal simply did not exist in the Comcast signal during motion intensive scenes. This may be responsible for the stutter and excessive motion blur seen with some video sequences on Comcast.”
…and the saga continues. Remeber all of the money you spent on that nice wall mounted HDTV set? Here’s an idea, take the money you are throwing away on Comcast and build up your BLURAY disc collection! (-;