I've been taught by the forum (and checked this out elsewhere) that for baseband audio signals (subwoofers, analog audio / RCA), RG59 is actually better than RG6. At least when we're talking about normal off-the-shelf cable. The difference is in the shielding / insulation, which is different between the two cable types. The typical construction of RG59 has better characteristics for low-frequency signals, while RG6 works better for high-frequencies (and is why it's required for high-bandwidth satellite and CATV).
The "best" thing to use are the RG6-grade wire made specifically for audio (or baseband video, not CATV), like the Belden Precision 1694a.
Now, that said, for short runs I don't think it will make any difference either way, and if there's a chance the wire will be used for other purposes, I'd probably stick with RG6.