Long story short, would definitely recommend the ES. We took the private couchette option: cheaper than the 1st class sleeper compartment (make up your own bunk with provided sheet, blanket and pillow, bring your own picnic) but you have your own space.

I mean, they're clearly 90s carriages and a bit rattly but perfectly clean and comfortable, as long as you don't mind sharing toilets/bathrooms (1st class sleepers are ensuite).