Yes, there's no diffinitive answer to a question like that.
I have taken cruises which made some stops in Greece, but have never done a true Greek Islands cruise so I don't think I am in a position to judge. We have, however, done a Baltic Cruise. Maybe it was the ship, the friends made before hand on a message board, the friend that cruised with us, but I consider this to be the best cruise we ever took. Seeing St. Petersburg was an experience of a lifetime. We were lucky because our cruise stayed in St. P. for three days.
What's nice about this problem is that there is no wrong answer.