As you need to spend quite a long time aboard, comfort does matter. Thus, the first-class deluxe sleeper cabins are better options for you. Bed linen, snacks, and hot meals, a hygienic set, bio-toilets – you will have most necessities aboard. However, consider the fact that there might be an issue with taking a shower. This is the other reason why breaking your trip might be a good idea.
In case you are desperate to feel the country’s vibe and get the unique experience of traveling by train as if you were a local, you may choose the second-class carriage. Less comfortable, but giving all the opportunities to connect with other people.
Finally, If communicating with Russian people is your priority, you may stick to the most budget option - the third-class carriage. Here you will be able to chat with your companions all day long, learning much info about Russian culture and its citizens.