Compared to Chimera, ChimeraX has a more modern user interface, better graphics, and much faster handling of large structures. For more information, see:
ChimeraX has several completely new features compared to Chimera, but not yet all of its commonly used capabilities. Some features are implemented as commands only, not (yet) as graphical interfaces. ChimeraX missing features relative to Chimera include:
...and many others. Chimera capabilities grew significantly over several years, and likewise, ChimeraX will contain more of these important features as development proceeds.
Although similar in many aspects, ChimeraX is not backward compatible with Chimera and does not read Chimera session files. Chimera includes an export to ChimeraX feature that covers some but not all types of data.