My family has done 7-night Disney cruises in the Mediterranean, Baltic, and Dover-Barcelona repositioning (as well as in the Caribbean) and enjoyed them all for different reasons.
If you have not visited Rome, Naples, and Florence but would like to, I would recommend the Med cruise. As the previous posted said, this cruise does require lengthy transit if you want to get to Rome and Florence, but those cities are definitely worth seeing at least once in your life.
If you want something less hectic, go with the Baltic. The main areas of Tallin, Helsinki, and Stockholm are all right close to the port, and all have unique sites. The port at St. Petersburg is industrial, so getting somewhere interesting does take a little while, but as with Italy, it's well worth the time.
We did the Dover-Barcelona repo cruise mainly because my husband wanted to see Portugal. Lisbon and Cadiz were interesting (and many main sites were walking distance from the port), but didn't have the wow factor of the cities we saw on our other European cruises. We did get a wow factor from Mont St. Michel in Normandy, although it was a drive of a few hours from the port in Le Havre which is also in the vicinity of the landing sites from D-Day. Disney offered excursions to Paris, but that would require an even longer drive.
We did not do the Northern European cruise to Norway, but that itinerary does sound similar to Alaska.
If you want the ship as much as the sites, a Transatlantic would work.
Really, you can't go wrong with any of the options. Just think about what sounds the most appealing to you.