I took this ferry (on Tallink Silja Line) in 2012.
If you reserve a cabin, you get the entire cabin. They will not assign other people to your cabin.
On their web site, when reserving a cabin, you'll see an option for "Special Cabins and Shared Places." If you want to share your space with others, that's where you can look.
As for paying more for one particular cabin class over another, I think this is no different from selecting a hotel room. If you are willing to spend more for a better/nicer room, go for it. If you'd prefer to spend your travel budget on other activities, go with the least expensive room that you feel you'd be comfortable in.