Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.

Waiting for the congregational prayer is better because the Prophet commanded his companions to delay certain prayers for joining congregation. He also asked the man to make prayer again when he visited the mosque and found them still prayer.

Moreover, the Prophet used to delay Ishah until his companions gather. Jabir bin Abdullah (may Allah be pleased with him and his father) said, “The Prophet used to pray Zhuhr at midday,  Asr when the sun was still bright, and Maghrib as soon as the sun sets. But, he would vary the time of Isha. When he found the people gathered, he would pray earlier; and, when he found them taking their time, he would delay it. The Prophet used to pray Fajr  when it was still dark.”

Almighty Allah knows best.