I’ve done the sleeper from Cornwall to London and back a few times. I really don’t recommend going in a normal seat unless you’re excellent at sleeping on flights, and the lack of proper curtains means you’ll get woken up when it does the 4am wait at Reading (which is surprisingly busy for 4am).
Getting a berth though is awesome. As long as you can sleep through the obvious movement the bed is surprisingly comfy, there’s a shower you can use in London, and in Cornwall you get woken up 30 minutes before your stop with a coffee and a bacon roll.