There's lots going on under the hood of both those motors, but if you end up having to look for used parts, there's a lot more chance of sourcing them for the Opti.
When buying new, I had a similar dilemma, which was easily resolved as the 4-stroke Yam F150 would have cost around £2500 more than the 150 Opti, so I bought the Opti.
It's a great motor when it's singing, but a PITA when pootling around at idle - very noisy, compared to a 4-stroke.
Suppose it depends on what you'll be using the boat for.
What boats are they on?