I travel on both Silja and Viking monthly. I live in Turku so I use the Viking Grace and the Baltic Princess ships. They are both evening ships and depart about 830 pm, arriving in Stockholm around 8 am. Turku has also two morning ships every day, 365 a year. I purchase my tickets directly form each ship's internet site and get great prices, often I pay 22 euros for a window cabin as a frequent passenger. Neither ship sells shared cabins. This sharing does not exist on the Turku Stockholm route. The cabin price is the same regardless if one or four people are in it.
As this route is shorter, the ships make the round trip in 23 hours, staying in port only an hour. I always sleep in and never leave the ship, instead I make it a mini cruise. I enjoy the ship's shows and shop. The Viking Grace is the newest and biggest and the cleanest (runs on LNG gas) of all the Baltic ships, with the largest tax free shop afloat in the world. The Baltic Princess is the same size, 2800 passengers, and has equally great shopping and entertainment. January sales are awesome!
The Turku Stockholm route is great as it has only 90 minutes of open sea, a fact to consider if it is stormy (Fall and Winter). First you have the beautiful Turku archipelago, then the Åland Islands, then the 90 minutes of open sea, and the rest is the Stockholm archipelago. Both ships are made in Finland, and one Christmas I took a special cruise to Riga on the Baltic Princess and we came home in 9.4 meter waves, safe and sound.
Both ships have a special Friday night and Saturday night departure policy that they do not sell alcohol on the outbound trip to keep the drunkenness at a minimum. The rest of the week is very peaceful and this rule does not apply.
Many tourists have found this route as it is a very inexpensive and fun way to spend a night and get to Stockholm. Many locals book several cruises back to back when they have some repairs done at home and they need to be absent, it is less expensive than any hotel!
I feel very privileged to live in a historical cultured city that has such fabulous ships leaving from my door step. I just love it!