Youngie, if you can get somewhere in Athens then it'll be more convenient, but Piraeus is just down the road, and i doubt you'll be going home in the early hours of the morning so no need to get a taxi etc...
Marc, i'd assume you want to be in Athens for the festivities / drinking. If you want a quieter start then i can't see stopping in Piraeus being a bad call, however i only know a couple of bars at the actual port. I'm sure one of the Olympiakos fans can guide better in regards to any good bars down there.