I have both. I think the ducted is quieter, but not by much. BUT my last 2 TT's that had ducted, also had the "blast" feature. This allows ALL the air to blast right out of the air conditioner straight down or around the sides of the unit. However, in "blast mode", it's just as loud as the non-ducted camper we had. I think the force of the air itself is what makes it so noisy. Going through the ducts, it gets disiminated into different directions and each individual ceiling vent has to be divided by the total number of vents, making the force of the air itself much quieter.
Both ducted camper were too long for the center blast only. But that center blast is REALLY nice when you've been outside on a very hot day and you come inside sweaty and exhausted. Stand under that thing then and it will freeze your toes!