Best position in a couchette

While not specifically relevant to the night jet (I've taken other night trains which I assume to be similar) :
Top has more privacy, but there is more train noise and it can be hot or cold depending on AC performance.
Bottom has more space.

Upper bunks on the trailing side of the compartment can be cold if there's no air conditioning and the window is left open for ventilation. I don't know whether there are still non-air-conditioned couchette cars in service.

Any advice on the best couchette position for the Nightjet, 6 berth? Thanks

Yes, from my comfy hotel room window watching the train leave town.

Just joking; Night trains and long ferries are something I gave up long ago. I found them uncomfortable, usually little sleep, and in a shared berth, a bit of a crap shoot as to what type of companions you get...though I have to say you meet some interesting people.

I am not discouraging you from your plans, have a great time, just could not resist the comment.

I don’t know your age, but being over 60 I would take the bottom berth. Wouldn’t want to not remember where I am in the middle of the night to go down the hall to the bathroom. Also, as mentioned, it has more room.

I prefer not to climb the ladder so I prefer the bottom bunk. Also if you get your luggage stored underneath, it's closer to you than others luggage might be to them.

You can see a small photo at https://www.seat61.com/trains-and-routes/nightjet.htm#Couchettes.