The 2 hp Yamaha is about 12 kilos (This is all from memory, i haven't sold Yamahas new for around 12 years) and the 3hp is 16.5 kilos.
The 3 however has a twist grip throttle and as said a gear box with forward and neutral.
Also the prop on the 3 hp is a splined bush instead of the 2hp runs a shear pin.
The 3 has a 2 clip cowling and it pops of for easy cleaning.
The 3 hp Yamaha rocks and is rather hard to beat. Hence why so many people use them..
I find the extra weight is a bit offset by the handles that make it easier to lift and carry, not as oar quad as the 2hp..
Hope that helps.
Quick edit: Also both these engines are designed to run on 100:1 fuel to oil ratio. So very smooth and quiet considering.