Let me go on the record stating that the Dometic Brisk Air is not quiet at all on the inside. That article must clearly be referring to outdoor noise (how I’ve seen it compared in the past) and it is indeed very quiet on the exterior. But boy it’s miserably loud on the inside.
I don’t wish to use anything less than a 13.5K BTU because a second AC does serve a redundant function. If the main AC failed during the heat of the summer, a 9K unit wouldn’t come to close to making the unit comfortable.
We don’t plan on using it at night. The main unit doesn’t have any trouble keeping up during the night time hours, so we would not use the 2nd AC at night. We put the fan on low during the night so that it always runs, which creates air flow and white noise. And for me, the unit turning on/off during the night would wake me.