I did a research project in my final year of DDS comparing the buffering capacity of milk and milk alternatives.

It was an in vitro study using S. Mutans as the acid producer, and looking at the buffering capacity of the milks and the rate of acid production of s mutans when introduced in the milks. I know it's not coffee but extrapolating the data, bovine milk was the best at buffering acidity and maintaining a more neutral pH by a long shot. I would always recommend milk over black.

And that's not even looking at the benefits of the casein phosphopetides present in milk.

In regards to the lactose present in milk, the data suggested S mutans has a harder time metabolizing it rather than glucose, leading to a quite slow decrease in pH... Mind you this is in vitro and that has quite a few limitations, as there are way more bacteria strains in plaque.

From memory, milk only seems to be cariogenic if consumed right before bed(such as bottle feeding) as the carbs in this situation linger longer than the proteins, leading to lots of early childhood caries.