Which is better: helping a son pay rent or buying food, and which has a greater reward?

Helping your son pay his rent is better than feeding strangers, especially if they are not in need, because charity given to a relative is both charity and the upholding of kinship ties. The Prophet's Sunnah has indicated the virtue of giving charity to relatives, and that relatives have more right to charity than others, and that one's spouse and children are most deserving of it.