I have a post for the Celebrity repositioning Spain to Miami.
The plus for Costa are the Caribbean ports of call rather than Celebrity's Canary Islands.
Other wise, Celebrity is the better bargain even thought the tax is $95 each.
Any comments on Costa? I heard the daily announcements can be annoying in multiple languages.
How to get to Genoa? From London Stansted Ryan Air on Friday the 4th for 3 GBP plus tax. No flight on Saturday.
Or Ryan air to Milan from several air hubs. Milan is not far from the cruise port, but research your trip in advance.