I built and flew my 9 for 7 years and have been flying a 12 for a year. The 12 is by far quieter and much smoother with less vibration as you would expect with a smaller, high rpm engine.
I have previously built two 8's and a 10. I have flown almost everything else including the 9 and 12, not yet flown the 3 and 14. There is no close comparison. The Rotax is more like a car engine, very quiet compared to the lycoming.
I built and flew my 9 for 7 years and have been flying a 12 for a year. The 12 is by far quieter and much smoother with less vibration as you would expect with a smaller, high rpm engine.