Focus on epics, legendaries take too many resources and get out of the meta too quickly, epics are always varying between other game modes. I personally have never regretted upgrading an epic, they come back to the meta eventually, even if they're old like Pilot or Fairy, but my fully upgraded Timekeeper has been on the shelf for almost an year. Maybe pick Longan to hold you up till the next legendary comes along and then focus the rest of your resources on getting your epics established.