Which camera is giving you better results when in actual use?
Understand first that any lens delivers to the focal plane a fixed amount of detail resolution, measured in line-pairs per millimeter. If the super lens delivers 100 line-pairs/millimeter, the 135 FF captures 24 * 100, while the APS-C captures 15 * 100. So FF will capture more total detail resolution, assuming both FF and APS-C have same pixel density (and all else being equal).
Secondly, if both FF and APS-C have same total pixel count, a single pixel is larger for FF, so it can gather more photons of light per unit time, so it will have better low noise characteristics than APS-C, all else being equal.
But not everyone needs 'the ultimate' all of the time, and if you are not a professional user, you might not need 'the ultimate' even most of the time. You might COVET 'the ultimate', but your prints might not need that in order to deliver fully satisfying photos.