If one had to be longer which would it be?


speaker cable or interconnect to amp?
128x128thegoldenear
In my many trials and tribulations, I have always found longer interconnects and shorter speaker cables sounded the best. I think I have always read that too. YMMV
It depends on your gear, including the wires themselves, how they'll be placed, and even your environment.
......and on the the impedance of the cables, the output impedance of the preamp and input impedance of the amp.....