For shipyards the difference is in the size of ships able to be produced. All ships may be built in shipyard space stations (see Space Stations Rules) if the ship is the length of the station or smaller (doesn't apply to stations 3km or larger). Obviously the large the shipyard the more stats it has as well (ie storage, passengers, and shields). As for Trade stations the Trading II has a larger capacity than the Trading I. Look over the statistics of the stations to see the specific differences between their stats.