I think both would be OK, but I prefer "on".
Student or Learner
Mexico is bordered by Belize and Guatemala to/on the south, and the United States to/on the north.
Do both prepositions work as well in this sentence?
(I got the above question and wonder if 'on' can also be used when I read this rule in the Collins Cobuild Usage:
You use `to' or `in' with the following nouns: east, north,
north-east, north-west, south, south-east, south-west, west
To the south-west lay the city. )
For me there's a slight change in focus, but the difference is so small as to be insignificant. If I were looking at map [in any country other than Mexico], I'd say 'on', if I were in Mexico I'd say 'to'.