Epic-starting items are hard to get.
Legendary-starting items are really hard to get. You can only get them from premium boxes. I don't even know if the missions in the game drop them as loot anymore.
And to answer your question, keep playing the highest-paying mission you got. Move up a difficulty if you can. You will have to even decide whether a longer-play (boss missions) will be better than a shorter-play (non-boss missions). Also, don't forget that playing daily gets you gold reward.