If one had to be longer which would it be?

speaker cable or interconnect to amp?
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.....
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.
Longer interconnect.
this has come up before you may want to check the archives
it would be a 25 foot run
IC.Can get a 25' run of decent quality for under a 100 made at most competent shops.Hifi Sound in Mpls. did one for me.I think I still have it.You can have it for cost.About 45$ if I remember.Good luck,Bob
Looking at your system, it seems like your "long" ic will be between your cd player and integrated. I would not ask a cd player to drive a 25' length. A preamp could and that would be my choice (most of the time) over long speaker cables but in your case, I would opt for the longer speaker wire.