ccache works at the translation unit level, caching object files. Zapcc caches (among other things) template instantiations between compilations.

If you are in a codebase that has some very complex header files with lots of (potentially nested) template instantiations, and then a large number of executables that all instantiate these templates, Zapcc can make a huge difference in compilation times. CCache doesn't really buy you anything in this scenario.

ccache does not speed up full build. zapcc does.

What do you mean by "full build"?

zapcc speeds up the build even if you changed a #define across the whole project or added a line to a header that you use everywhere leading to recompiling of all your .cpp files.