Has anyone taken the train from Tallin to St. Petersburg? Is this a better option than the ferry to Helsinki and the faster train? Is the higher price for higher class travel worth it on the train? Is the scenery beautiful?
I just took the ferry from Helsinki to St. Petersburg and back as a part of the 72 hour, visa-free ferry option. It worked very well for me and I had a very easy trip, so I would recommend it. I wrote about the process of booking it on my blog, check it out and let me know if you have questions: http://katehaswanderlust.com/2014/06/the-rooftops-of-st-petersburg/
I took the bus from St Petersburg to Tallinn and the scenery along the land route is, IMO, "meh"!. I found the scenery in Russia to be pretty depressing, the towns and homes in that area are poor and not well maintained. I wasn't surprised by it as I had just spent a week touring northern rural Russia. In Estonia, however, the farms, homes, in general seem much more prosperous. BTW, The bus was a horrible experience, but the once a day train schedule didn't come close to our schedule.
It sounds like you are asking if the direct train from Tallinn to St Petersburg is a better option than taking the ferry from Tallinn to Helsinki and then the high speed train from Helsinki to St Petersburg. I think the latter is a more interesting option but it would certainly be much more expensive, and depending on schedules and wait times, could take twice as long.
As to paying the differential between first and second class on a train, it's a personal choice, if the few differences are worth the extra euro cost to you. Like RS says, second class arrives at the same time as first class.