Fast Lens or Bigger Sensor?

I would personally go for a larger sensor: the larger the better. Since there is no FF sensor in a mirrorless camera (except M9 which is too expensive), I would go for a PAS-C camera.

I really don't understand why someone would long for Leica m9 (unless, he is a billionaire).  It is too expensive and the sensor may not be that great for its premium price. It does not make your photos any better. It's true that it's currently the only FF rangefinder (mirrorless) in the market. If I wanted to pay that much, I would probably buy a medium format camera instead. There are rumors Sony will come up with a mirrorless FF camera in not a too far future. I will most likely buy one when it is out if they don't screw something with it (like a too bad user interface, or else)

The most recent sensors are better than the older generation not only in regards to high iso performance but also for their dynamic range. I would not go for a Nex 5 or older Olympus/Panasonic models. If buying a Nex, I would probably wait for Nex6/5R reviews and if their sensor is no better than Nex 5n or 7, I would pick up a 5n or a 7. Or, Fuji X1 Pro, which in my opinion has a nicer body design (retro-style). The good thing about Nex 5n or 5r is that they are relatively inexpensive and leave you some money for acquiring some nice lenses to go with.