Is it true that Allah differentiated between His servants in creation, but not inفضل (favor/preference), and are there any Shar'i (Islamic legal) proofs for that?

Allah Almighty made the true distinction among people based on piety, and the most honored among them in His sight is the most pious. As for the disparities among people in this world, such as in sustenance and health, they are for a wisdom known to Allah. This does not imply a superiority of one over another in Allah's balance. Rather, the pious poor person may be better than the rich, and the grateful rich person may be superior if he is upright in his religion.