We will be in Helsinki during the week of August 19 and would like to take a day trip to Tallinn. How much in advance do we need to buy ferry tickets, and is it worth the extra 60€ for 'business class'?
I was there several years ago, bought tickets an hour before the ferry left and had no trouble. For a 100 minute trip, it wasn't worth it for me to go business class. They're already much more comfortable than your standard airplane. We took the Silja line, as I recall. Have a good trip!
Unless it's the last ferry of the day or on a major holiday or some such, no need for advanced tickets. You can always buy a day ahead if you wish a particular time. No need for business class, they are 90 minute rides and are comfy. You can walk around during the ride. I think if you buy round trip, you can get a small discount and it saves you time on the return.