Amp output=>speaker circuits are low impedance. Therefore, use the SHORTEST and THICKEST copper wire you can reasonably handle. Make sure your connections are always clean and tight. These two considerations alone will make a more audible difference than whether you pay 50 cents a foot versus $50 a foot for cable. The idea is to minimize the series impedances so that the voltage that appears at the speaker end of the cable is as close as absolutely possible to what the amplifier puts into it. If possible, twist the two conductors together. That will make the speaker cable less apt to act as an antenna--that is, pick up RF interference and conduct it into the amplifier. Some amp designs are more immune than others to RFI, but it's a worthwhile precaution nonetheless.
