Neither. Both have issues and getting them fixed are insanely expensive.
My boss has a newer Mercedes and it is ALWAYS in the shop. I've known people with Infinitis who get screwed on repairs and maintenance too, plus a lot of the G-35s have a starting issue. Even simple things like headlight bulbs need to be sold with some KY on these cars.
I'd go for a Chrysler 300 or a Cadillac STS. They're be easier and cheaper to own and maintain, give you comparable or better reliability, cost less at purchase and in the long run while still giving you good performance (and an aftermarket to improve performance) and in my opinion, they look a lot better too.
Even if the transmission goes out in the Chrysler, which is bound to happen eventually, you'll still have more money than if you had to get a brakejob on one of the other cars.
