When building a factory city a dry dock is a really useful facility because it holds a lot but doesn't interfere with city design too much, as it can be placed inline with the PGs, allowing you to have 12 factories in the city. A starport, because of its width, will mean that you can't fit as many factories and I think it uses more power too.
A starport may demand more room and power, but it can store 2,000,000 units (not tons) compared to the dry dock's 150,000 capacity. It all really depends on how many units of raw materials you plan to store. For reference, 2 million units of just meleenium would be 22 million tons, or 22 full Tabders/Mammoths. Starports are king.