its hoenestly about "prefrence" and "Functionaltiy" what type of gun is it for? what is that guns primary job? (IE: range, speed, general, DMR, CQB, ect) and then you play style.
but if your aiming for High RPMs and are willing to loose some FPS for it, aim for High Speed Motors. look at some of the systema models for speed. and bravo make a damn good one if you ever see it in stock. if you want the speed and the range/fps one of the best options is the Sytema A-Z motor for speed and range.
its up to you, but i do have a brand new Systema A-Z with a ever newer Guarder Gear on it for durability. pm for more info