Ok, it's interesting to see how Faction XP actually works.
You get 1 xp for each Dumont, Janisar or Dryden delivery made. Distance or amount of freight doesn't effect the amount of xp.
So if you were strictly after faction xp, you're better off hauling as many small loads of Dumont as you can carry and go for the shortest distance you can find. ... and it's worst to look for large Dryden loads that take 5 minutes to deliver, because you still only get 1 xp.
Miners don't seem to make faction xp for mining, but they do received 5 xp for each trade commodity that they find (and sell) while mining.