XLR-RCA-BNC sonic quality differences

For long runs I would think BNC would be a bad way to go because usually BNC connectors are used for video coax and it is the wrong impedence for audio signals.

The main thing about XLRs is the lower noise floor and less potential for hum. When used in a truly balanced system you also have the advantage of 6db more of gain. An XLR uses three conductors. The positive and negative have there own conductors and the ground floats above and below the signal.