Quality of HDMI cable does make a difference as long as you are watching high definition video, like for example a blu-ray disc through a good player like Oppo on a high-definition screen. It does practically nothing otherwise.
Preferred cables are those with silver plated copper center conductor which transmits the video signal. What comes into play here is the loop resistance at baseband video frequencies, especially in HD transmission. An example, solid copper conductor is preferred in CCTV transmission where distances are long and other materials like copper-plated steel create this problem. The advantage of silver plating is that the transmission of data is better as HD frequencies are supposed to travel closer to the surface. At the same time, the quality of copper is equally important. However, silver in audio side conductors has to be considered with caution as it easily induces listening fatigue.
Any HDMI cable with around 4% silver, 6 will be even better, will certainly outperform the others in terms of picture clarity, speed, black level etc. Of course, your system should be capable of differentiating them.
You don't have to spend a fortune to get such good cables. I believe Pangea cables (audioadvisor.com) have a good reputation.
I own a Wireworld Silver Starlight 5^2 HDMI cable and use it for only video transmission between a Oppo blu-ray player and Pioneer Kuro plasma. It makes a big difference compared to the stock cable.
I do own silver interconnects, Audioquest Sky and Cheetah in my audio system (CDP - preamp - power amp). But they are special and clean and fast with no tendency for listening fatigue. It all depends.
Lastly, the Wireworld is the best value for the money as they have some of the best HDMI connectors. Several cables have issues with the connectors and often affect the performance.
Just my 2 cents, please, and no controversy intended.
cheers.
murali