Free Spectral Range versus Axial Mode Spacing?

I am doing research on micro cavity frequency combs and I've been trying to understand things like dispersion, Free Spectral range, resonance, and modes.

I am looking at the definition of free spectral range(FSR), and the definition of it seems to be the frequency spacing of a resonators axial modes.

So when would you use one versus the other? The mathematical definition for FSR is:

$$\Delta\lambda=\frac{\lambda^2}{n_gL}$$

While for resonators axial modes is:

$$\Delta\nu=\frac{c}{2n_gL}$$

Where $$n_g$$ is the index of refraction of the medium, L is the length of cavity, c is the speed of light, $$\lambda$$ is the wavelength, and $$\nu$$ is the frequency.