>I think FX files are generally of higher quality as they need >less enlarging to any size of output based on its larger size >of the sensor.
I'm afraid that is not really the case.
The relationship that matters in this context is between the pixel dimensions of your image and the print size you want. A 12MP image will give you the same print resolution whether it was captured on a DX or FX sensor.
Where the FX sensor scores is that, for a given pixel count each photosite will be larger, with benefits to things like dynamic range, noise and tone/colour rendition. Put another way, a 10.5MP DX camera using the same sensor technology and firmware as a D600 (if such a device existed) would give identical image quality.