Have traveled many times to Antigua from San Ignacio. There is a small new bus terminal just on the other side of the river bridge going into Melchor de Minco...the name escapes me, but it is on the right side of road just several doors from the river. Anyway, the bus leaves about 9PM and arrives in Guatemala City about 5AM. From there, I usually take a taxi to board "chicken bus" to Antigua. The taxi driver will know where to find it. The bus is basically an express bus...although it has no restrooms and is abit shabby. Also from Flores...a first class AC/restroom bus travels from 10PM to 6AM to Guatemala City. Much better bus. Good luck!!