Go with Silja, Viking has a reputation for being a "party boat". A few years back I rode Silja from Stockholm to Helsinki. Midway through the journey the boats from both companies dock at some island for a short time. When the Viking boat pulled along side our ship, everyone on the Viking boat was drunk stupid: yelling obscenities, and throwing beer bottles in our direction.
It's an amazing journey, one of my most memorable travel experiences! The scenery is stunning as the ship navigates around the seemingly endless archipelago outside of Stockholm. I was upgraded to one of the expensive cabin classes, forget the exact category but the room was fantastic; there are picture and descriptions of the cabins on the website. When you get sick of the scenery there are a lot of activates around the ship: shopping, casino, cinema, bars, restaurants. Live entertainment was a Vegas style magic show. The early show was family friendly, the late night version was the same except the female assistants were topless. Between the two shows the room was a disco with a live band.