Full face snorkeling masks vs traditional snorkel masks
One of the main reasons people enjoy full face snorkel masks so much is because they allow you breathe through both the mouth and nose. This such a more natural way of breathing.
Fogging and leaking are the biggest complaints with traditional snorkel masks. Sometimes, no matter what you try, you can't stop a conventional mask from fogging.
With the new full face snorkel masks, the fogging is either eliminated completely or drastically reduced (depending on the brand). These masks cover your entire face but are split into two separate chambers. The upper chamber is the viewing section. The second chamber covers both your mouth and nose and is connected to the snorkel piece. Fogging can be seen in the lower area, which is ok as it's separate from the viewing section. Air is drawn through the snorkel piece and circulated through this bottom section.
Just like a traditional snorkel mask, full face masks leak if it's not the correct size for your face. This is the most important thing when choosing a mask. The size needs to be correct - and the company you purchase from should have a good returns policy in case you need to swap it for a different size.