It's good to keep a mix of missions to lessons so you ego never gets high. I made the mistake of letting my ego get up to 65% and it took me forever to get it down to under 10%. To me the best way to go is always be swopping out a few missions then do some lessons according to the ego you gained from the missions. It takes longer at the moment to build up your starbits but in the long run it is actually faster and you get 120 xp for each lesson so you are still gaining levels to allow you to move onto new missions.