I would go with os. They break in really quick because of the percision, I have never had a problem except with their .46 la. There is apparently something about that engine becaue i have heard other stories about it while the .40 has had good, but not excellent feedback. Go with the .61 fx. They are incredible engines. My fx has never died on me with the exception of an empty tank. Os engines also have great longevity. I know some guys at the field with the cheaper predecessor to the la series, the fp. One of them at the field is about 20 years old, and has not developed any blow-by.