What is better? longer interconnects or spkr cable

Which ever is cheaper - they both have tradeoff's. 15' of speaker cable is really not a long run and would probably be my first choice - but by using balanced interconnects you do avoid one of the problems with long runs of IC, that is noise and you do get you amps away from the other components which could help you to avoid heat and other noise problems.

I've studied this problem analytically. The results show that with TYPICAL amps and preamps (SS) and cables that are in the same quality range that the optimum ratio of speaker cables to IC's is between 3:1 and 1:1. Typically you cannot go wrong with 2:1. See the Audio FAQ page of my website for the analysis:
http://www.empiricalaudio.com

If you have poorly designed speaker cables that are of a high nominal impedance, you may make gains by going to long runs of balanced interconnects and keeping the speaker cable short. Otherwise, i would suggest using short interconnects and long speaker cable or a combo that is a median point.

Just bare in mind that long interconnects typically have little to no resale value where the opposite is true of speaker cables. I have purchased 20+ foot pairs of interconnects for amounts equal to or less than a standard 1 meter pair would cost. I was able to do this because the seller could not give them away. Having done that, i cut them to the lengths i needed and re-terminated, making out like a bandit. In some cases, i was actually able to sell the left-over cabling that i did not need for what i paid for the whole lot. Sean
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Sean,excellent idea..thats using the old noddle..