The real way to great sound from wires? Use the Swedish blond virgin nymphos as the wire supports to elevate the cables off the floor. Have one lie naked on the floor every couple of feet, and drape the wire across their torsos so it doesn't touch the ground at any point. You know, just the way Japanese sushi gets served on a girl's naked stomach in some of those exclusive clubs. You could experiment with different configurations, of course, such as spread-eagled and holding the wires clamped between their toes.
In all honesty, I have never noticed even the slightest sonic degradation under such conditions.
Seriously, the obvious answer has already been given in this thread a few times. Try one wire, then try the other one, and then try them both together. The best way to do this would be to let a friend make the swaps in a way that you can't see which wire or wires are connected. Let your ears decide for you. Swap back and forth a few times, and then if you hear a consistent difference, choose the best one. If there is no difference, then it won't matter which you use.
Diameter really matters only when it is too small. Extra diameter is good, but the difference isn't always audible.
I did an experiment once using elevator power cables as speaker wires. The individual copper strands were over 1/8" thick, and the whole stranded cable (just the metal part, without insulation!) was around 1-1/2" thick. Of course, the ends had to be trimmed down with a jerry-rigged connector just to be able to attach the cable at all. Its diameter completely dwarfed the entire connector(s). Honestly, the sound was miraculously transformed, as if the angels themselves had descended from heaven to sing through my speakers --in my dreams. In the real world, the difference between those ship-sized cables and my normal ones was minimal, if there was any at all. It was a wee bit disappointing. Thought I might start a DIY trend, but alas...
[Note: if you are one of those people whose ego is tied up in the size of your cables, look for elevator motor cables - I guarantee you'll have the biggest audio cables on your block. Probably in your entire State! Oh, be prepared for some seriously heavy metal-bending work to get them to line up, too. At that thickness, they're almost impossible to bend very sharply!]