Ive heard good things about the os.18 but i would assume that the 3.3 is most likely the faster engine simply because of displacement(i know that more displacement doesnt mean more speed) and what traxxas has said about it. although, from what ive heard, traxxas does have a reputation for engines that are fairly difficult to tune and tend to overheat so i wouldnt recomend you go out and but the 3.3. buy the os.18 is probably the better all around engine.