I've taken trips on both Viking and Silja, possibly on the same ships you mention because they have both been around for a while. In the past, Silja ships were considered to be nicer but I don't know if that is still true. Silja was sold to Tallink a few years back and I've heard that there is not as much of a difference now. The Silja ship has a large promenade in the center which makes the ship feel more open.
Both companies follow the same route, so scenery will be the same. In Stockholm, Viking docks at Stadsgården which is about a 15 minute walk from the Slussen tunnelbana station, a very centrally located station. Silja docks at Värtahamnen (Gärdet is closest station).
Viking used to attract more of a party crowd because they didn't require you to book a cabin so they got some people that just stayed up all night drinking and dancing; I don't know if that (no cabin) is still permitted.
The ferries are quieter on weekdays than on weekends. Fridays and Saturdays have a higher proportion of drinkers and that applies to both lines. Keep in mind, these are ferries and not "real" cruise ships. They both stop in the Åland Islands in the middle of the night (not part of EU) and this allows them to sell duty free liquor. There is a certain crowd that travels only for that purpose.
I'd avoid the cheapest cabins which are below the car deck.
For what it is worth, I'm traveling this route in August and I will probably go with Silja.