If one had to be longer which would it be?

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.....

how can I find this all out sounds intimidating

It is intimidating, and possibly expensive. How long are we talking about? You CD player is single ended (not balanced) and might have issues with powering the longer cables to your integrated. My first guess on this is try to longer speaker cables unless you are thinking more than 15'/5m.

I would tend to agree with Mofimadness unless you are talking very long runs for the interconnects. I use 2 meter interconnects from my preamp to my monoblock power amps which sit next to the speakers. My speaker cables are less than one meter and I am very pleased with the results.