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.