OS motors are generally the easiest to tune, but they don't have as much power as some of the Piccos (and other similar motors).'
It really depends on your willingness to tune the motor.
I have an OS RG and it was pretty fast. I put the OZ RZ piston and sleeve in and the thing is VERY fast. I'm up with the Picco's right now on power, with the ease of tuning the OS offers.