I voted 7.
I played one game at 5, the first one after I finished the tutorial, but I didn't feel very committed to it and gave up.
Every game since then I have played on 7. I have not completed a game yet, I have lost a few and in one of my solo games I am 11 hours into it with 7 systems under my belt. It seems to be going well.
There is also a co-op game with a friend that is going well, that is on level 7.
I have experienced such a variety of challenges at level 7 that I have no urge to 'upgrade', but then I am pretty new to this game.