Bigger birds are more forgiving of input errors due to inertial mass. My T-rex 250 has a mass of 369g, while my T-rex 500 has a mass of 1874g. With five times the mass, the reaction time will be one fifth that of the 250. In other words, my reaction time has to be 5 times faster to fly the 250 correctly.
This also assumes approximately the same thrust-to-weight ratios and the same magnitude of cyclic and pitch throw. As far as dealing with turbulence and other sources of external force, i.e. not transmitter input, the 5X ratio is spot on. This is somewhat negated, however, if you have a 3-axis gyro like the smaller CP helis have, but they will still be much twitchier and will easily piro fast enough to become a blur. I found this out when I lost my tail on my 250, lol.
The upshot of low inertial mass, however, is that crashes are much cheaper. There simply isn't enough momentum while doing simple hovering and slow forward flight in a smaller bird to do anything serious like break the frame, etc. Put a 600 into the ground with enough force, and you are likely to trash just about everything on the airframe.
The 120sr does seem to be a nice bird. It is much more stable than my 250 in the simulator that I use, but I believe that is due to the 3-axis gyro. What Tony said is true though, fixed pitch would be the next logical step, but going right to CP is not out of the question. My first heli was a CP, and I learned the hard way. But we're all in it for the challenge, right?
