Dear Shiva — this is actually a very important question. I doubt anyone knows the answer. As you know , it is in general advisable to breath through the nose (prewarming the air, moistening the air) — but some people do not do it for all kinds of reasons, and must breath through the mouth.

As to the coronavirus SARS-Cov2 docking site, the ACE2 protein, there is to my knowledge no data available on its presence in the mouth and pharynx. The mouth cells are different from that of the airways, but some parts of the mucosa cells in the pharynx (throat) are very similar to those on the nose.

I expect that the pharynx (and perhaps even the mouth) ALSO has ACE2 proteins (for developmental reasons), and thus could be also a site for virus entry.

Therefore, in brief, the answer to your questions is that most likely, THERE IS NO MUCH of a DIFFERENCE between mouth and nose breathing with respect to risk of infection with the SARS-Cov2 virus. The important thing is to protect both orifices from droplets! So whatever mask you use, it should reach high to cover your nose.