I'd look at itinerary over line in this case. The Baltics are usually quite port intensive. Be sure the cruise you pick includes at least two days in St. Petersburg and also be sure you have a day in Tallinn. Estonia was a sleeper and a real highlight.
I tend not to cruise R.C.C.L. just because of food quality and ship furnishings that seem to be lacking in comfort level and finish details, but the others should all be great if the itinerary and prices meet your budget.
We were on the Oceania Regatta for a Baltic cruise in June. It started in Stockholm and ended in Dover. We went to Helsinki, Tallin, St. Petersburg(3 full days), Warnemunde, Germany, Gdansk, Poland, Amsterdam, Bruges. It was a 14 day cruise and was absolutely wonderful. The ship holds only 684 people and we truly enjoyed our cruise.
We did Celebrity to the Baltic last summer and had a wonderful time! One thing Celebrity does that the other lines do not is offer free shuttle service into the city centers. Other lines charge $10-20 each way, which really is a rip-off. I rank their food and service higher than RC and NCL, but their entertainment lower. I would definitely recommend this cruise.