Is it better for someone who prays 'Isha in congregation late with the Imam—while he is praying Tarawih—than for someone who prays it at the beginning of its time? And what is the ruling on saying, "Pray for me"?

Performing the ‘Ishā’ prayer behind the ‘Ishā’ congregation is better than delaying it and performing it with the Tarāwīḥ congregation. It is not permissible to recommend delaying it for that reason. The validity of performing the ‘Ishā’ prayer behind someone who is praying Tarāwīḥ is a matter of disagreement, and the majority opinion is that it is not valid.