From my experience, arena gives more cash per hour spent.
You can generate extra medals in arena easily by modifying your play style.
i.e. in the beginning rounds, throw grenades near the unarmored enemies to knock them down, then kill them with your gun to earn an easy $250 While They're Down medal.
Also, shade kill cash just goes to the person that makes the final kill shot, so it can frequently become just a game of people racing to pick up the power weapons. Not a very co-operative spirit that game mode encourages...