Airport is further out and about 50eu to the port. I'd definitely recommend coming in the day prior. I've done the train and metro both with luggage and wouldn't recommend it honestly. Take a taxi to your hotel and then a taxi the following day to the port. Taxi for airport to Athens city is about 40eu and probably about 30eu to the port from the city. You can also get with a company like PK travel if you have a late flight leaving and they can do a city tour and then drop you at the airport at the end.