There's one more transport option for getting to and from Tallinn, and it's for the more adventurous, those in a mad panic rush, or those with money to spare. The Copterline helicopter trip from Tallinn to Helsinki takes just 15 minutes, and if you book carefully can be had for as little as 89 euros. This works out as about the same as a flight across the Finnish Gulf, so is definitely worth considering.
The Copterline flights take off from the end of Linnahall, to the north of the old center, at the end of the pier next to the Linda Lines fast ferry boats.
From Helsinki, the easiest way into Tallinn is definitely by helicopter. Check out Copterline's website for last minute deals that are hardly any more expensive than speedboat services into Tallinn from Helsinki, except that the helicopter takes you there in 18 minutes instead of 100 minutes by speedboat.