BNC is a purpose designed 75 ohm connector for video (and digital) signals. It is cheap, well designed and work REALLY well. RCA is a connector designed for a completely different job. It doesnt work as well (for anything) and is generally more expensive but is widely used in the domestic arena. Yes you can tell the difference between the two, quite easily and this difference becomes more marked with high bandwidth signals (computers, PS DVD players, scalers etc) but IS visible with ordinary interlaced signals. Professionals will ALWAYS use BNCs.