I just returned from a trip to Helsinki, and stayed at the Scandic Grand Marina. I really liked this hotel. It is in an old warehouse, but inside is completely modern. The room I stayed in had a very comfortable bed and a good working bathroom. You get free use of the hotel sauna, and there are three computers with internet access free for use by the guests. In addition breakfast is free (and good). The hotel is right across from the cruise ship docks, but there was not really that much extra noise. An added bonus is watching the cruise ships bound for Stockholm pull into and out of the port.
The hotel is also within walking distance to downtown, many good restaurants, and the train station. In addition, you are about 400 meters from the ferry to Suominlinna, the Swedish built island fortress. Whatever you do, make time to get out there and explore that. The round trip ferry ride is a little over 2 euros, I think.
The hotel is about 20 minutes from the airport, but the drive is a little tricky if you are not familiar with Swedish / Finnish road signs and names. Getting back to the airport is simple (follow the airplane signs on the street signs.)
Have a good trip. Finland is fun. Be sure to try the Finnish beers and get to an authentic style Finnish restaurant at least once.