Who is more deserving of having one's seat vacated for them on public transportation: an elderly Muslim man or an elderly Christian woman?

Your act of standing to seat an elderly person is a commendable deed of righteousness and benevolence. Allah bestows His mercy upon His merciful servants. A Muslim is more deserving of righteousness, benevolence, and mercy than a disbeliever, and it is better to prioritize the Muslim in this regard, as the Prophet, peace and blessings be upon him, said: "Indeed, part of glorifying Allah is honoring an old Muslim."