Cheap is the most important point. Why would you not to buy a cheaper one. In my experience, it is a very fast engine and comparing the prices and performance of 12 class engines, LL3 is the best buy.
仲有好多因素, 天氣, 濕度, 火咀, 油( 係外國,歐洲 .12 engin 主要用16%), engin 調教, 曲pipe 配合, clut ...
Justinwan ching's opinion is right as the performance of an engine will be affected by the set up of clutch (shoe and spring thereof), the two speed transmission (earlier or later to open the clutch), all bearing of your rc car and engine, model and size of exhaust pipe, etc...