I need to read more about Mercury.I am not sure about difference in sound/loudness between those two specific motors. However, I would really consider rbtnt’s point above. The Mercury 400 would be the most ideal for wieight to HP if considering that motor range (unless there is a dealer/service center reason for sticking with the Yamaha). Also, the higher HP Mercuries (at least in the 200-350 range) run slightly quieter. I am assuming those above 300 do as well.