GCC 5 in Fedora (Red Hat developer blog)

The symbol versioning approach makes random crashes rather unlikely. (AFAICS, they can occur only if something in C's public API takes a pointer/reference to one of the stl structures whose ABI changed, and P uses that feature of C's API.)

The soname versioning approach makes random crashes virtually guaranteed, because ld.so will happily pull in both sonames at runtime, and then everything in P's internals, everyting in C's internals, and worst of all, libstdc++ own internals will all be exposed to incompatible ABIs. However, in this case, the crash's cause will be easier to diagnose for a moderately sophisticated user (as long as P doesn't have a binary plugin infrastructure).