O.S is by far the better choice ! cheaper in$$ ,more power ,more durable, and most of all very tuner friendly.. The .18 hpi costs 30$ more and only has a 5.5 mm carb ( 99% of .12 & .15's uses a 5.5) the o.s .18 uses a 6.5 mm carb ( so that extra 1 mm = more fuel/air = more power)..
Just ask any one about a o.s engine period! Great power , long lasting hp beast.. can't go wrong.:breakdown