On July 13th I took Silja Serenade from Helsinki to Stockholm. They are the guys that invented the promenade in the middle of a cruise ship which now most major cruise ships have. So the Silja vessels are more like cruise ships than like a ferry, yet they operate every day on a very punctual schedule. It's like a resort-hotel that changes its position over night. You start in the middle of the one city and the next morning arrive in the middle of another one. Journey time is 17 hours and it's actually too short given all the entertainment options - and the fact that you may want to sleep for 6 or 8 hours as well ;-)
Should I take the cruise from Stockholm to Helsinki or from Helsinki to Stockholm?
Vessels depart at 5pm local time, you can embark from 3.30pm and start with the roof-top spa (sitting in a hot tub enjoying the view over the city etc.) or whatever else. If you depart from Stockholm the first 3 hours of the cruise are going to be through the archipelago and are absolutely scenic. The next morning in Helsinki only the last 30 minutes are actually scenic, so you have plenty of time for your breakfast. If you depart from Helsinki the first 30 minutes will be scenic and you should consider maybe an early breakfast the next morning so you can enjoy the sail through the archipelago for at least the last 60-90 minutes.
What kind of cabin should I book?
The choice is from a 4-bunkbeds interior cabin to a luxury exterior suite. The thing is: You will only need your cabin as a luggage storage room, to sleep and to take a shower. If you're sailing in June and July please note it doesn't really get dark at all. If you're a light sleeper any exterior cabin (with window or porthole) might be too bright for you at night. On the other hand if you cannot handle being in a closed box with no windows - go for the exterior cabin. All the cabins from Deluxe and up have recently been renovated and look like nice 4 star hotel rooms. The A2 or A4 exterior cabins still look like on a ferry boat but are clean, comfortable - although I would suggest not to squeece 4 people into one of those. 2 is fine though.
The Tallinksilja website wants me to pre-book meals. Do I want that?
Despite the website promising you to save some money over buying on-board that's not true. Also if you prepay your breakfast you still have to wait in line with all those people who pay as they go. My clear recommendation: There is so much choice on board, wait until you see it and pick whatever you like. The Grand Buffet by the way has a nice and big selection BUT it's packed with travel groups and just not the nicest experience IMHO.
Keep in mind that Helsinki is in Eastern European Time whereas Stockholm is in Central European Time. The departure and arrival times on the website are local times. Both countries are Schengen countries. Currency on board is €uro and of course all major credit and debit cards are accepted. There is a slow but free Wi-Fi but given you're on the sea don't expect any mircales.
UPDATE: On the Helsinki side the tram goes directly to the ferry terminal and it's very easy to use it around the city. A ticket valid for 80 minutes is €2.90 and can be purchased via app or at kiosks. On the Stockholm side there is construction in the ferry port. The easiest to do to get there is to buy the Flybusarna charter bus ticket from the central station to the Tallink Silja terminal for SEK 600 oneway. The public bus is "only" SEK 420 but the bus stop is maybe a 500 yards walk via a covered walkway-bridge from the terminal.