I tend to look at overall points standings. Over the course of the entire season, when it comes to on track exploits, Rosberg is the daddy of that team. Of course, I think we all know, off track, MSc is the big man of that team.
If Merc did produce the best car, MSc may need to arrange for Rosberg to drive slow or pull over, to get the wins (something which he may do). Don't get me wrong, MSc will finish ahead of Rosberg on some occasions, but Rosberg is likely to outscore MSc, over the course of the entire season (based on the last 2 seasons).