Long interconnect or short Speaker Cables

From Blue Jeans Cables:
"Speaker cable is a bit different from a lot of the interconnect cables we handle, in several respects. Because speakers are driven at low impedance (typically 4 or 8 ohms) and high current, speaker cables are, for all practical purposes, immune from interference from EMI or RFI, so shielding isn't required. The low impedance of the circuit also tips the balance of concern from capacitance, which is important in interconnect use, to inductance, which, while a concern, can be controlled only to a limited degree. The biggest issue in speaker cables, from the point of view of sound quality, is simply conductivity; the lower the resistance of the cable, the lower the contribution of the speaker cable's resistance to the damping factor, and the flatter the frequency response will be. While one can spend thousands of dollars on exotic speaker cable, in the end analysis, it's the sheer conductivity of the cable, and (barring a really odd design, which may introduce various undesirable effects) little else that matters. The answer to keeping conductivity high is simple: the larger the wire, the lower the resistance, and the higher the conductivity"

As simple as that!!!!

There is no right answer. In all probability, there will only be a very subtle difference in either scenarie.... most likely this difference will be inaudible. So go with whatever setup is most convenient.

You might take a look at Alpha-Core Goertz MI2 speaker cables. I have been using them for about 3 years and been extremely pleased. In your setup, the A-C Goertz speaker cable has two virtues:
1. the cable has very low impedance, so longer runs are not a problem;
2. Alpha-Core states on their web site that you can use uneven lengths of their speaker cable without causing problems.

Alpha-Core offers a 30-day money-back trial if you order from their web site, so you could try them to see if you are satisfied. Here's the link to their web page for the MI series of speaker cables:
http://www.alphacore.com/mispeaker.html

As others have said, it depends on your specific system and electronics, and differences may range from subtle to clearly audible. If the pre and amp are fully balanced, this is a no-brainer, use the long IC's and short speaker cables due to the noise rejecting benefits of balanced cables. However, 3M would be my suggestion for the practical limit of a single-ended interconnect before risking noise problems (once again - system and cable dependent). Regarding speaker cable lengths, for practical distances of about 2M to 6M, some say up to about a 50% difference in length may not be a problem. One 12-foot run and one 18-foot run should not make an audible difference, and is feasible if you are using bulk cabling. However, if buying commercial cables you may want to consider equal runs to facilitate future resale. If you have significant extra length in one cable, don't coil it.

Forgot to add, a lot of people like the Goertz cables described by Sdcampbell above. They do have low inductance, which is desirable in a speaker cable. However, they also have very high capacitance compared to many other cables, and their use at your lengths could create problems for your amps unless you use the zobel networks recommended by Alpha-Core (and available from Alpha-Core).