Is one of the following scenarios a better choice on average?
1] On each planet one plays all the missions and every thing to do before going to the next planet or
2] On each planet one plays many of the things to do including missions but leaves some things to do for later since there will be someone (an NPC) who is asking one to do something off world that seemingly cannot wait. The player also would go to the next planet and start some things to do before all the things are accomplished at the last planet.
Your scenario 2 doesn't actually work very well. I thought this might be true when I was leveling my first character, but it just isn't. Planet have specific levels for which they are appropriate. If you move to a new planet to do missions you will find one of two scenarios are true. Either: a) you are under the level for mission on the new planet (which makes it tough to do), or b) you will return to finish the missions on the old planet and find you are way over-leveled (which means you get very little experience for completing them).
The intent seems to be to finish one planet at a time and then move on completely. Some of the bonus series have you returning to planets, but mostly not.