I live in Ireland and have been to both.
the port stop in Cork is actually Cobh, a small rural fishing port about 20 minutes from Cork city. The Cork area is rural countryside, rolling green fields. Cork is in the Republic of Ireland, so the currency is Euros
Belfast is a more urban city. It is a very historical city. The main rural attraction is Game of Thrones which is filmed locally. It has alot of political history. Belfast is technically part of The UK, so the currency is pound sterling.
Both ports are also Titanic ports. Belfast is where Titanic was built and there is now an amazing museum at the actual real shipyard where Titanic was built.
Cobh was the last port of call of Titanic and there is also a museum and walking tours.