What's better: RCA or XLR?

Thanks to all who responded. the technical viewpoint is relevant, however, my main concern is, as someone suggested, the difference in the "quality of the sound". There seems to be so many different views on both sides of the question. My only experience with cables is this. I have a pair of Harmonic Tech XLR pro-silway from amp to pre, and it replaced a cheap set of rca's. I heard no difference, although I must admit I didn't do a lot of critical listening comparing the 2. However, when I replaced the cheap rca's from cd player to pre-amp with a set of Cardas quadlink 5c RCA ends there was a noticeable improvement.
thanks, joe

Very good clarification by bear regarding the XLR vs RCA connector. I would further clarify that XLR I/Os could be installed on the chassis of any component that was unbalanced in nature, and simply bridge the pin configurations for true unbalanced operation. That being said, I feel that the RCA connector was a very poor design that quickly deteriorates after repeated use. The lack of any locking mechanisms, to insure stability and reliability, seem rather amatuerish as an industry standard to me. Its no wonder the original Mark Levinson gear insisted on Fischer/Camac type connectors. He saw the problem and adressed it.It was unrealistic, since all other equipment lacked these, so interface was always through more RCAs. It was like having a 24 bit machine with only 16 bit discs to play through it.............Frank

I concur with Saki winki.

While it is usually best to have a fully balanced system I did get rid of a hum problem that I had with my amp by putting XLR's on the amp end of my interconnects and leaving RCA's on the preamp end. I eventually added XLR outputs to my preamp. Even though it is not a balanced design, I like the better connection that the XLR's make.

I had similar set-up about 6 months ago definitely go with the XLR it is more quiet background and dynamic.