If I miss the last rak'ah, is it better to complete the prayer individually with the Imam, or to consider it a supererogatory prayer and pray with another congregation?

It is better to join the Imam and then make up what was missed, based on the saying of the Prophet, peace and blessings be upon him: "Whatever you catch, pray, and whatever you miss, complete." Whoever catches the Imam in the last Tashahhud is not considered to have caught the congregation, but he will receive a reward commensurate with what he caught. One is considered to have caught the congregation if he catches at least one Rak'ah, based on the saying of the Prophet, peace and blessings be upon him: "Whoever catches a Rak'ah of the prayer has caught the prayer."