Is it better to pray supererogatory prayers at work than in congregation at the mosque when time is short, or vice versa?

Congregational prayer in the mosque is better than praying individually, due to the generality of texts indicating this. These include: the hadith of Abu Hurayrah (may Allah be pleased with him): "When the prayer is called, there is no prayer except the prescribed one"; the hadith of Ibn Mas'ud (may Allah be pleased with him): "Whoever would love to meet Allah tomorrow as a Muslim, then let him regularly observe these prayers from where they are called out"; the hadith of the blind man whom the Prophet (peace and blessings of Allah be upon him) commanded to answer the call; and the hadith of Ubayy ibn Ka'b (may Allah be pleased with him) regarding the virtue of taking many steps towards the mosque. One can combine the two obligations by praying the obligatory prayer in the mosque and performing the regular Sunnah prayers whenever he is able.