Done both and I would do the Baltics first and then the Black Sea later. The Baltics and St. Petersburg is really quite amazing. The Black Sea is very interesting but a destination for those who have already done the Med and the Baltics and just want something different. If you are talking sea conditions - six of one and half a dozen of the other. Not a scientific point of view or based on any knowledge. YOu can hit rough seas or calm seas anywhere and to my knowledge (which is limited other than sailing in these areas) neither area is known for rough seas.