Both. You're describing cosmological redshift when you talk about the expansion of space increasing the wavelength of light. You're describing Doppler redshift when you talk about light signals exchanged by objects that are moving apart because they have different velocities.
They're both valid ways of producing redshifts, and you need to think about both of them (as well as gravitational redshift) when you consider the redshift that occur in the light of distant astronomical objects, like galaxies and quasars.

Grant Hutchison