Is my act considered valid if I perform the Tarawih prayer without having prayed the 'Isha prayer in congregation, or would it have been better for me to pray 'Isha with them with the intention of a supererogatory prayer?

It would have been better and more preferable for you to pray the Isha (night) prayer with the congregation with the intention of it being a nafilah (supererogatory prayer), based on the hadith of Yazid ibn al-Aswad: "When you have prayed in your dwellings, and then you come to a congregational mosque, pray with them, for it will be a nafilah for you." And based on the saying of the Prophet, peace be upon him: "Pray the prayer at its appointed time. If it is established (the iqama is called) while you are in the mosque, then pray, and do not say: 'I have already prayed, so I will not pray.'"