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.
Entertainment: Advantage Crystal. Entertainment on the SS was enthusiastic but basic.
Service: Both excellent but more uneven on Silver Spirit, ranging from the superb to the chaotic breakfast buffet at Terrazza. Butler service tops, but so is regular room service.
Food: Both excellent. Favorites on the SS were Indochine, Grill and Terrazza dinner, as well as Silver Note. Atlantide was fine. Our one serious disappointment was SS's Japanese restaurant (extra $) - much of the food was unauthentic and drowning in teryaki, much better in Crystal's Nobu. I also missed my main dining room on the Crystal Serenity.
Wines: advantage Crystal. The offerings on SS were fine but on CS I had some excellent Loire, Burgundy and Cote de Rone wines. You can get this on Silversea but pay extra.
Embarkation/Disembarkation: Advantage Silversea. Crystal is good, Silversea is better. Silversea's policy to provide shuttles to the center of each town where it docks is perfect.
Bottom line: we would be very happy to sail again one either ship/ with either cruise line!!