I cannot think of anyone who uses both solutions at the same time, and why would anyone want to? If you link against StubsG it will run everywhere without any worries about CLib versions etc. If that works for you, why would you even want to look for alternatives?
The latest releases by ROL have narrowed the gap considerably. The C99 enabled CLib in the A9Home allows executables to run that previously required the Castle CLib. The C99 enabled StubsG allows programs to link against StubsG that previously required the Castle Stubs (and hence the Castle CLib on 26-bit systems).
So, with these new releases, it matters less and less which linking solution a program uses, both solutions are converging.