This is all heresay, but are my opinion. People that seem to know imply the Cadet 3 is a copy of the Drake. The Drake and the Vespin use the same flush mechanism, but the Vespin uses the Unifit adapter. The adapter is not embedded in the toilet, and could be louder than the Drake. I've put Drakes in my mother's house, and have a Vespin and Carlyle in mine (mostly because I like the skirted design which makes cleaning easier). All of these make sort of a gulp at the end when the suction is broken, ending the flush.
The pressure assist is likely to be the noisiest of those three - just the quick release of the pressure can be startling to some people, and is nothing close to subtle. I don't think there is any toilet that is truely quiet except maybe an outhouse - but the flies and mosquitoes may exclude that from consideration!
There are probably a few people here that can give a valid personal comparison. Personally, I'd stay away from power assisted flush mechnisms...noisey, and take more maintenance (and you'll probably not find the parts everwhere like you can for most others, either).