I have the 3W 56iB2, Above idle it is very smooth. Best engine there is for making an airframe last a long time. Draw back will be building an exhaust. I make my own 2 into one headers and run single cann exhaust. This engine really likes being on tuned exhaust. It can and mine does weigh just under 4# after a bit of modifying.
Next choice for power would be the 3W 55xi, Pure brute power and light.
Next choice BME 58 Extreme, Another screamer and the lightest possible weight. Both these will shake some but not too bad. Either of these would be excelent, because they are top of the line and will fit any application in the future.
And if you don't need lots of power the BME 50cc Echo conversion engine runs and idles very smooth. Acceptable sound from a stock muffler. Super easy to mount with the beams on the crank case. Keeping the weight down with a glass filled mount is a snap. Next best choice for long airframe life. Just not a good choice for a future 3D aerobatic plane.