You will get the OS fans that will say OS is the only one to get. I have them both, OS has a track record that is a mile long & they are a top of the line engine & I have had them for years. Evolution is fairly new but they came out running hard. I have 1 - .45 Evo. 2 - .46 Evo's & 2 - .61 Evos. They are all Great engines. In my opinion they are both good engines so I think the question is how much do you want to spend .