If you've never worked or tuned Nitro Engines, I'd highly suggest you get the O.S. It costs a little more, but there's nothing more frustrating then getting to the field and not being able to fly because you bought a cheap engine and can't get it tuned correctly. Nothing is as easy to get up and running as an O.S. If you are a beginner, I'd suggest starting with an O.S. and then once you have some experience tuning engines, trying some different brands. You can find used O.S engines on the cheap in craigslist or ebay.