This is really a question of what look you are after and your budget. Colorbond has zincalume steel as its base and is then painted with multi-layer, patented methods that have been tried and tested in Australian conditions.
Zincalume is a fantastic product but Colorbond is far superior in terms of its durability and look. It comes in a variety of 20 different colours, from almost white to black.
Zincalume has that silver steel roof colour that many people like and it is slightly cheaper, which is why it is used more commonly on commercial buildings with large roofs.