You really can't go wrong with either. It really depends on what you want to spend and how comfortable you will be with whichever engine you choose. I have raced both Novarossi and OS engines and have not had issues with either. Some out here will swear by this engine or that engine; many never having tried the other so they would not know if any of the competition is comparable. The one good thing about OS is that their internals (piston/sleeve and rod) are cheaper on the pocketbook than Novarossi. So, pick the one you will feel most comfortable and roll with it.