Dr.EM said:
As far as audio goes, you want the lowest resistance cable you can get, without it being too good for the equiptment. I've heard that when setting up a hi-fi system, you should spend 10% budget on cables. I also think its more important with speaker cables as they carry more current.
You have no need to spend silly money on cables, it's just a big scam, with the so called 'Monster' cables often performing worse than ones a fraction of the cost.
For speaker cables it's a good idea to use a reasonable thickness, particularly for long runs - but it's nothing to get too excited about. For signal cables the main difference is the amount of screening provided, if you are in a noisey environment you might need to use decent quality screened cables (particularly for microphones).
But most expensive cables are a complete waste of time, never buy anything that says 'monster', 'oxygen free', 'gold plated' etc.
The resistance of copper is very low, and for signal cables it makes very little difference (due to the impedances involved), but for high power speakers a thicker cable will help to reduce any loss - mains cable is as good as anything else for this.
If you want to compare different cables, the only way is to get someone to change cables repeatedly for you, while you do blind listening tests. The person changing the cables writes a list of which ones he used, which he randomly changes about - and you write a list of scores you give the different cables.
I'll be VERY surprised if you can reliably detect a particular cable, or even tell any difference at all!.