heliman
X-plane may have great visuals, but the helicopter model has some serious issues - any movement of the cyclic causes huge changes in the torque/ manifold pressure, for example.
MD FS X is probably OK for piston engine helos, but won't properly model power limiting (by slowing the rotor down when you reach maximum power).
Both need large visuals and full size controls to really get to see the effects of controls, etc.