Last year I would have said OS 50 with out batting an eye, however I took a chance and bought the TT 50 and wow what a great engine. Very smooth very powerfull, just shy of the OS 50 in power and this is a first hand comparison comparing apples to apples, same heli, same fuel, same muffler and on and on. It runs smoother than the OS 50 and has never quit on me in the air or on the ground, my friend has had his OS 50 quit in flight and on the ground he's still having problems with the idle to.