Both will probably be full when I depart (looking at the Russian railways booking System).
I'm not interested in talking to fellow passengers (esp. since my Russian consists of but a few words and phrases) and just want to relax, and sleep during the nights (I'll also bring earplugs).
Some people say kupe is calmer because there are only 4 People in it, but others say that, because it's more closed, the noise will be more irritating and the neighbors are more likely to try over-socializing.
Definitely not the platzkart! I'm serious, you'll never get a calm trip if you are in there. Platzkart has additional side places across the train, so it's up to 52 people there in a car compared with 4 * 9 = 36 people in kupe car.
Also, if you have an upper place (the even one), you simply can go upstairs, and rest for whole trip (yes, there is a probability that your neighbors will try to speak with you, but it's much less than in platzkart. Also in kupe you have doors to close and remove outer noises, which you can't do in platzkart.
I've done both several times (as well as obshi class in Kazakhstan/Uzbekistan - but that's a whole different story).
I've had one crazy trip in kupe - my first. There were Russians, and they had vodka. And I was inexperienced. It was rough.
However, even then, when I was under the weather, I found the top bunk is the best. You can retreat there, and they'll leave you alone if you want to be.
Other trips I experienced the same. During the day, the bottom bunks tend to be for eating/socialising, so if you're on the bottom, you're going to be part of the group without a choice.
The same applies to platzkart. I actually really liked platzkart for the social aspect, but still prefer the top bunk so I can retreat when need be. On the top (I was ill from Murmansk to St Petersburg), people realise when you need time to yourself, and might greet you when they enter your area, but that's about it.
Platzkart will almost certainly be noisier, but both are a fine experience and a wonderful cultural eye-opener, however I'll always try for the top bunk regardless if I return there.
