We just returned from a very enjoyable Baltic cruise. We are not Cruise experts; this was only our second one (recently) after Alaska cruise with Crystal Serenity. We naturally compared the two lines/ships. Overall we loved both but do have a few nitpicks.
Cabin: SS clearly more spacious than Crystal Serenity but the the CS balcony cabin was better organized and we preferred Crystal's less firm mattress. SS walk in closet is great.
Public spaces: Advantage Silver Spirit. Beautiful pastel colors and rich style. Crystal Serenity is certainly excellent in its own right, but it has some garish or frumpy spots.