Unless you need to dramatically upsize an image, Genuine Fractals is a poor interpolation tool. Photoshop bicubic will do a better job without the odd fractal artifacts when simply doubling file resolution (up to about 4X upsampling). I reserve the use of GF for upsampling of 4X or more. That's where the fractal artifacts are preferrable to the pixel magnification that becomes obvious via bicubic interpolation.
There's more to resolution than just the physical pixel count. Good, controlled resolution chart tests using the same lens and aperture settings on the two cameras should be telling as to how well the pixel data is handled by the camera's processing to resolve lines.
With digital imaging from chips that have RGB sensor grids, there is always interpolation to fill in the missing data for the colors not sensed at a given pixel. There is also antialiasing processing and noise reduction going on. All of these can have an impact on the end resolution in lines per inch/mm.