Silja line or Viking stockholm to helsinki roundtrip

I took Viking from Stockholm to Turku last year. The Viking dock in Stockholm (Stadsgården, on Sodermalm) is easier to reach than the Tallink Silja dock (Värtahamnen). Info in my notes--perhaps from this forum--indicates that if you're heading to the Tallink Silja dock, the Flygbussarna charter bus is more convenient thanthe public bus. It seems there's a lengthy walk (over 1/4 mile) from the public-bus stop.

However, even figuring out how to get to Stadsgarden can be a bit tricky. I ended up going to Slussen (there's a Metro station there), taking the elevator down to shore level, and taking a bus that went past the terminal and then let me off. I had planned just to walk to the terminal, but my effort to cross the bus lot near the foot of the Slussen elevator sent a bus-company employee chasing after me. He really, really wanted me to take the bus, so I did. I had a multi-day transit pass at that point, else that little bus ride would have cost me over $4, and I'd have been annoyed. (I'm assuming the guy didn't direct me to some sort of free shuttle bus; it didn't look like one.)

I don't know about the situation on the Helsinki end since I disembarked in Turku and later took the train to Helsinki, but I vaguely remember thinking that the Tallink Silja dock is more convenient in Helsinki.

I don't know about the luggage situation. I wouldn't assume the ship is just going to sit in Helsinki all day, but I suppose it's possible that's what it does.

Is there a benefit to one line over the other re: location of the
ports in either city or on board.

One big benefit is that Silja has daily departures. Viking sold one ship recently so now they only depart every 2nd day. Apart from that, there is really not a huge difference. Viking docks bit more central in Stockholm, and Silja docks a bit more central in Finland. Some people like Silja's ships with their "promenade".

Also, was wondering if the round trip cruises allow you to keep your
luggage in the cabin while exploring Helsinki during the day?

Usually you can do that.

Also, if anyone knows a customer service email for Silja line, cannot
seem to find one.

https://en.tallink.com/contact-us

The Viking dock in Stockholm (Stadsgården, on Sodermalm) is easier to
reach than the Tallink Silja dock (Värtahamnen). Info in my
notes--perhaps from this forum--indicates that if you're heading to
the Tallink Silja dock, the Flygbussarna charter bus is more
convenient thanthe public bus. It seems there's a lengthy walk (over
1/4 mile) from the public-bus stop.

To get to Värtahamnen, a popular way is to take the metro to Gärdet and walk from there. An easy walk, at least as long as the weather is nice. It's about 800 m though, so if you have a lot of luggage or it's raining a lot stay on the metro on more stop to Ropsten and change to bus 76 that stops closer to the terminal. Or take the transfer bus that stops outside the terminal, but on the other hand costs 75 kr for a single trip.

I had planned just to walk to the terminal, but my effort to cross the
bus lot near the foot of the Slussen elevator sent a bus-company
employee chasing after me. He really, really wanted me to take the
bus, so I did.

It used to be very easy to walk from the metro station, but Slussen is a bit of a mess at the moment with all the construction.

I don't know about the luggage situation. I wouldn't assume the ship
is just going to sit in Helsinki all day, but I suppose it's possible
that's what it does.

That's exactly what it does.

It looks like Viking will have daily departures in the high season if this press release is still accurate.
https://www.vikingline.com/press-room/FB85B85B08D12B59

I’ve taken both lines over the years, but my last trip was in 2014 (on Viking) so it has been a while. The interior of the Silja ships was more impressive (the promenade that Badger mentioned), but the services and cabins were similar. Depending on your date of travel, you may not have a choice given that you want to do a cruise.

Thank you for all the responses - I was confused by Silja because the ship to Helsinki is the Symphony but it looks like the return ship is called Serenade, so I thought they change ships (maybe I am not booking properly for the RT cruise?). Viking looks like they do want you to take luggage off.

I was confused by Silja because the ship to Helsinki is the Symphony but it looks like the return ship is called Serenade

I think you might be doing something wrong or interpreting the page wrong. Make sure you book the ”Cruise” option rather than the ”Route Trip”. You should be on either the Symphony or the Serenade. Make sure you are using European Date format. For example, if you are traveling from Stockholm to Helsinki on August 6th and returning on August 7th, your dates would be 06.08.2023 for the outbound trip and 07.08.2023 for the return trip.

See this page
https://en.tallink.com/40-hour-roundtrip-cruise-from/to-stockholm-helsinki

While the graphic shows both ships, if you book the cruise the ship you take will depend on the day of travel.

It certainly looks like Viking will return to daily departures. And since the ships will not stay in port during the day I would not count on being able to leave things in the cabin.

And if you are booking a cruise, both trips should be on the same ship. Either Silja Serenade or Silja Symphony.