PCB vs. Cluster
Choosing depends on the application. It's really quite simple.

PCB stands for "printed circuit board" This means that the pixels are affixed to large panels typically in configurations of 8 x 8 or 16 x 16 pixels. LED signs comprised of PCB's are usually covered by a solid Lexan face material in order to protect them from the elements and increase visibility.

Clusters are groups of small led's that are combined, usually in a cylindrical device, which is connected to a circuit board. Clusters are individually removable, much like light bulbs and are usually shielded from sunlight by a hood or louver.

When installing a LED sign in a location where it will be close to the ground and may be easily reached a PCB with a solid lexan cover is preferred. It is much more vandal resistant as well as aesthetically pleasing.

If your LED sign will be installed higher in the air where there is little likelihood of anyone touching or tampering with it, cluster technology is the way to go. Clusters are considerably brighter which is good for longer range viewing. Also when character sizes get larger PCB's become impractical.

Now that we have determined the type of LED sign, we can address the method of transmission.