HARD CHOICES
You should be happy with either...
I chose O.S. for my first 4-stroke, and have been very pleased with it. However, there are several people running 4-strokes at the local club field, and the Saitos have been just as reliable. It would be difficult to rate one above the other.
The YS's seem to need a bit more tweeking... but the people running them are putting a LOT more time on thier engines, practicing for Patern competition. I imagine that if I was burning 1 to 2 gallons of fuel a week, my O.S. would need a bit more mantenance too.