Power cord - 5 ft vs 10 ft ?

as short as possible.  as long as necessary for comfortable routing.  Avoid coils and loops.  

these are rules of thumb that are based on physics.  that said. you probably won't hear a differnce between 5' and 10' if both are good quality heavy gauge (10 awg) cable.

I like the way @hilde45 put it.  The size of the conductors is 99% of it.  I would much rather have an ebay chinese 10 awg power cord than a high end 14 awg power cord.  If fact, I would promptly replace the latter.