depends what you can afford.as a beginner you want a reliable engine.in the past os was the engine to give trouble free service.also ball bearings are better than a bushed engine but you must use after run oil so they do not rust.a twin needle carb is harder to set at first but i prefer them for the low idle reliability over the air bleed carbs.the 2 needle may have difficulty in the midrange but once set the problem is gone.usually i do not have to fool with needle settings.fill,start and fly.i would lean to the os fx or a magnum.good luck