Really depends on what you are interested in. The Greek Isles are designed for cruising and you can get a good idea of what they are like even if you are only in for a day. Their written history predates that of the Baltics, so there is a lot to see.
I haven't done a Baltic cruise, but I have been to FInland and Russia. I think that you get more of a feel for the culture when you do those countries by land.