You should go with the better cable. In my experience there is not much of a difference between a single and biwire pair of the same type cable; ie, two single Type 6 would be about the same as the Type 6 Bi-Wire version.
Other reasons to get the single cable are: (1) that some audiophiles use a different type cable for the high and low frequency speaker posts; (2) If you have a budget of say $300 for cables, you could buy a $300 single cable and save up go get another $300 single cable later, instead of buying a $300 Bi-Wire cable or two $150 singles. If you start with a single to save for a second single later you will need a jumper wire to the other speaker post.