I think NM_CACHED_QUARK_FCN() is better because:

- the implementation is in our hand, meaning it is clear that
  putting a "static" before NM_CACHED_QUARK_FCN() is guaranteed to
  work -- without relying on G_DEFINE_QUARK() to be defined in a way
  that this works (in fact, we currently never do that and instead
  make all functions non-static).
- it does not construct function names by appending "_quark".
  Thus you can grep for the entire function name and finding
  the place where it is implemented.
- same with the stings, where the new macro doesn't stringify the
  argument, which is less surpising. Again, now you can grep
  for the string including the double quoting.

(yes, I really use grep to understand the source-code)