  The purpose of this paper is to present a comparison between the modified
nonlinear Schr\"odinger (MNLS) equation and the focusing and defocusing
variants of the (unmodified) nonlinear Schr\"odinger (NLS) equation in the
semiclassical limit. We describe aspects of the limiting dynamics and discuss
how the nature of the dynamics is evident theoretically through
inverse-scattering and noncommutative steepest descent methods. The main
message is that, depending on initial data, the MNLS equation can behave either
like the defocusing NLS equation, like the focusing NLS equation (in both cases
the analogy is asymptotically accurate in the semiclassical limit when the NLS
equation is posed with appropriately modified initial data), or like an
interesting mixture of the two. In the latter case, we identify a feature of
the dynamics analogous to a sonic line in gas dynamics, a free boundary
separating subsonic flow from supersonic flow.
