I have seen both 'resonant frequency' and 'resonance frequency' used in scientific literature. But which one, actually, is correct: 'resonant frequency' or 'resonance frequency'?
As you noted yourself, both terms are in use (and actually yield similar amounts of Google hits).
However, in my eyes the term that should be used is 'resonance frequency', referring to the frequency at which resonance occurs. 'Resonant frequency', i.e. the frequency at which something is resonant, strikes me as a very unusual structure and I struggle to come up with examples following this concept.
See also: http://users.ece.gatech.edu/mleach/misc/resonance.html, an opinion pretty much mirroring my own.
As a native German speaker, I can add to that that in German the term is "Resonanzfrequenz" and not "resonante Frequenz", which, of course, boils down to the same discussion in the end.
Both are correct but have different meanings.
Resonant frequency means a frequency which is deep clear and continuing to sound or reverberate.
Here resonant is an adjective and modify the noun frequency.
Resonance frequency means the value of a frequency when the resonance occurs.
Here the resonance is a noun and has an attributive relation with frequency.
Several answers have touched on this point, but I'll try to dive it home. The term resonant (adjective) frequency (noun) implies that it the frequency that is resonating. More likely, you mean to describe the frequency of resonance of some system. That's why "resonance frequency" would be the better choice.
Never the less, "resonant frequency" is widely enough used to qualify as an idiom. No one will think less of you for using it.
They both seem fine to me. One of the meanings of resonant is simply "pertaining to resonance", so I don't see there is a problem describing a frequency as being resonant.
Personally I like resonant frequency more than resonance frequency. It just sounds better to me, but that's just personal preference.