The word clock generator sends digital pulses to the other device so that it can keep in sync with the incoming digital audio samples. In other words, so that it reads the samples at the same rate/speed as they are being sent. The word clock pules are read completely independently of the incoming stream of samples. If the receiving device was allowed to read at a different speed, it would likely miss a "bit" here and there which would cause errors to occur.
I don't believe it makes any difference which type of cable you use, as long as it is in good condition and clean. BNC cables are basically the same as RCA audio cables but with different ends. It can be used for either digital or analog signal. TOSlink (optical) delivers simple pulses of light which are converted into digital signal. It must also be kept free of dust at the ends and never folded or knotted.
If you already use optical for the digital signal itself, that is one less cable you need to plug in, if the word clock comes through the same cable.
I believe both work equally well as long as the sending clock has a good strong and stable signal.
