The 120 is just a larger msr. Not too much larger...but if you plan on flying outside...the larger one will serve you better as these are FP helis and dont fly that well in the wind. They do fly better than any co-axial in wind...but really they are for calm flying.

If you are really serious about learning to fly "real" model RC helis...just start out with a CP heli. Blade makes 2 great ones that are tough as nails. the Nano CPX and the mCPX. They are about the same heli but the Nano is smaller...its about the size of the MSR. The mCPX is slightly larger....but not as large as the 120 size. Id call it a 110 size even though i dont think that size really exists. Also, get a decent SIM to practice on first. I use Clear View and it hasnt let me down. I knew nothing about helis and have no mentors in my club as I am the only heli pilot and I used it to teach myself to fly. It just simply works.

goodluck