I've just noticed another ferry company, namely Eckerö Line, serving between Helsinki-Talinn. I guess Tallink and Viking are more reputable and therefore widely discussed in tripadvisor forums.
The thing is that for the dates I am looking there is a substantial price difference between Eckerö Line and the others. For instance, for a one-way Talinn-Helsinki trip Eckerö Line is EUR 24 per adult whereas it is EUR 31-47 at Tallink and EUR 45 at Viking.
What is the reason for this? Anything fishy? Do they all berth at the same terminal in Tallinn? (As a side note, the price difference with Viking is even more interesting because they are very similar as they both have two trips each day, unlike Tallink which runs 7 trips starting from early morning until midnight)