Is it better for someone who did not catch the first row in congregational obligatory prayer to leave the first congregation with the regular Imam and wait for the second congregation in the same mosque, or to pray in the second row?

It is legislated for whoever enters the mosque and finds the Imam praying to join him immediately, and not to wait for another congregation. This is by consensus of the scholars.

This is supported by prophetic hadiths, among them: - "When one of you comes to prayer and the Imam is in a state, let him do as the Imam does." - "When you come to prayer and we are in prostration, prostrate, and do not count it as anything. And whoever catches the bowing, he has caught the prayer." - "When you come to prayer, observe tranquility. Whatever you catch, pray, and whatever you miss, complete."

Therefore, it is preferable to join the resident Imam in the first congregational prayer of the mosque in any situation, due to the virtues therein, such as a larger congregation and not missing the reward of the first congregation, even if one does not reach the first row.