G'day science man, I went from a yammie 2 stroke to a mercury optimal to a yammie hpdi and now own a yammie 4 stroke 150.
The 4 stroke is by far and away the quietest and cleanest engine I have ever owned and for that reason would never go back. Yes the opti and hpdi had more performance on my boat however nothing that noticeable. It is a no brainer to me and would only ever buy a 4 stroke from here in in. Originally when I went to the 4 stroke, I uses to try and turn the boat on thinking it was off because it was so quiet oly to realise it was on. It has never missed a beat and I've done. 2 seasons with it and approximately 250 hours and couldn't be happier. Any questions, feel free to ask.