To actually answer your question, no the OS is not worth the extra $$. NONE of their 2 strokes are IMO. Their 4 strokes are a much better value: great engine/not too cheap but a reasonable price for the engine you get. Of your two choices I'd go with the ST, though I would pick a ThunderTiger over an ST. Less tweaking=more flying and they are great engines overall. (though I have been meaning to try out the Evolution 61, maybe that'll be my next engine…)