noise nmos
I think that the most popular explanation for the lower flicker noise of the PMOS is because the PMOS i s implemented into separate wells which decreases its flicker noise.
Concerning which will have lower noise, PMOS or NMOS, there's no exact answer for that question. It depends on the circiut, transistor sizing and the frequency of interest. For example, If you're designing an amplifier, for lower frequency offsets, the noise of PMOS may be better than NMOS because of flicker noise. However, at higher frequency offsets, the input referred noise of the NMOS will be better because of the higher gm. If you're designing a current mirror, the output noise current will be better for a PMOS than an NMOS for the same sizing because of lower gm.