I concur with the answers. You already have a BMW and you already know how it rides and you already know about the maintenance. The infiniti would be a more practical option only because of the miles. You know atleast with the infiniti, you have atleast another 30K miles before it needs major servicing. The BMW's and Merc's are on the higher side of mileage and that means maintenance.
But you also know the Inifiniti will be boring compared to the BMW's. I personally don't like the Merc only because that body style just looks ugly. It almost looks like the designers were drunk and just started sculpting the clay while robotripping.
Now if you are mechanically inclined, I'd do the BMW's in a heartbeat. I always think working on cars is a stress reliever, for me atleast. If you know how to turn a wrench and don't mind using a saturday to pull the valve covers and replace gaskets and such and trace spotty oil leaks, I guarantee you Sunday will leave you with a smile.
Can't really say that with infiniti.
