Train tickets are never cheaper on the spot, usually they are more expensive, sometimes much more so, although on this route the price should not change very much so if you are comfortable with paying €34, you don't need to worry. As @neo explained, there are also some special tickets that might allow you to save some money, especially if travelling with a group (although it might not be evident at first, this regional ticket is valid all the way to Poland and in local buses and tramways from ZDiTM Szczecin, see this page in German for details).
Regarding ride-sharing, it's a bit more work to organise and you need some flexibility. For example, if you catch a ride with someone coming from far away and driving through Berlin, you need to agree on a meeting point, they might be running late, etc.
In practice, you are simply riding in someone's car so it's as (un)comfortable as car travel generally is. On blablacar in particular, you can see the car make and model and how many seats are offered by the driver (more often than not, the middle seat in the back of a 5-seater will be left empty) or check the reviews to have a more precise idea of the level of comfort you can expect with a particular driver.
Beside the low price, another advantage is that you often have interesting discussions but if that's not something you are looking forward to, then ride-sharing is probably not a very good option.
I don't know anything about buses.