I would say a 17.5 motor because you can get faster and control it easier. If you drop a crazy fast motor in the car then it's gonna be hard to learn how to drive it. Remember even though 17.5 is slow you can always turn it up with boost and timing or once you get used to it upgrade or change to a faster motor. A lot of people make the mistake and say they want to go fast but when they go to race they have to race a class they don't have a chance at winning and they can't control the car. Slower is faster most of the time anyways.