Is it permissible to leave a prayer in progress to join another congregational prayer, or is it better to complete the individual prayer?

It is permissible, according to the preponderant opinion, for one performing an obligatory prayer (fard) to be led by someone performing a supererogatory prayer (nafilah). There is no harm in interrupting the prayer to attain the reward of congregational prayer. In some cases, it is commendable to change an obligatory prayer into a supererogatory one, such as when a person begins an obligatory prayer alone and then a congregation arrives. In this situation, he may change his prayer into a supererogatory one, complete it with two rak'ahs, and then join the congregation. Alternatively, he may interrupt it to catch the congregation if he fears missing it. Interrupting an obligatory prayer in this case is not forbidden; rather, it may be commanded. As for how to exit the prayer, the taslim (salutation of peace) is valid, and it is permissible to exit without a taslim.