I do not know about the TransSiberian, but I have taken the Red Train from Saint Petersburg to Moscow [and back]. It was expensive, but worth it. Our own cabin with drinks & a meal. [it leaves at midnight!] Obviously not much to see on the way, but I love train travel so it worked quite well for me.
On my next journey, I would like to take the day trip. It still takes about 8 hours, but more to see, I am sure.
In my own experience, one gets what one pays for, so, do the best you can afford!!!