Undisclosed, it more depends on the distance. RG59 is typically suitable for distances up to 750 feet or 1,000 feet, depending on who you ask. RG6 is suitable for up to 1,000/1,500 feet, again depending on who you ask. Then, there's RG11, which can transport video even farther.
For video, that assumes all coaxial cable is copper/copper (100% pure copper center conductor plus 100% pure copper woven shield with at least 95% coverage).
By the way, SDI has more stringent cable requirements due to its high bit rate, but the transport distance limit increases with larger cable sizes still apply.
I don't know of a reason you couldn't use RG6 in place of RG59. Same goes for using CAT6 in place of CAT5e.
IPVMU Certified | 02/06/15 01:35am
One of the reasons I asked was I read this article which overall is saying RG6 is superior, until the very end:
Coaxial cables, commonly called ‘coax’, are the cables that are commonly used for video and satellites installations. There are many kinds of cables to suit different applications, among which are the RG59 and RG6. The main difference between the two is their construction as the RG6 is bigger compared to the RG59. The inner conductor core of the RG6 cable is thicker as well as the shielding within the rubber sheath. The difference between the two, in terms of construction, results in a better frequency range for the RG6 cable. RG6 cables are often certified to work up to 3Ghz.
Although RG59 cables may work above the 2Ghz level, it is not always the case and problems may still appear later as the conditions change. Another key difference is the much lower attenuation of RG6 cables. With very long lengths of cable, the attenuation can degrade the signal to a point where it becomes noticeable. Using an RG6 cable simply means that you are able to use longer cables before you have problems with attenuation. A downside to the RG6 cable is when it is used for applications operate below 50Mhz. A good example of this is HDTV, which only operates at around 37Mhz. The shielding of RG6 cables cannot properly prevent noise at levels of below 50Mhz and the signal may be worse with an RG6 cable than with an RG56.
Not sure what the frenquency of good old NTSC is but I thought it was pretty low. Not to say that I trust this article by any means, and if you all say it's always better, that's good enough for me...