If it were XP that I was after, I think I would rather trade for the gems. Then use the gems to buy a ship that will heavily Indio me with XP.
A side note;
The gems help me get a better ship...
The better ship helps me acquire resources quicker...
....also deliver resources quicker...
Resources delivered quicker inturn gives me XP faster.
...So not only do I get XP for purchasing the better ship, I also get XP for being able to perform tasks at faster rate. - - Sorry! - I believe gems will always be a better trade in the long term regardless of your end goal.