I'd go with 12, or maybe 14. I bought a nice 250 foot spool from monoprice and stopped worrying. My wires are inwall generally, and it's nice to set it and forget 'em. Do they sound better, I would highly doubt it. But sometimes the mere fact that there are more substatial
is nice.

Btw, my longest run- to patio, zone three is about 60, maybe 70 feet.