This is a hell of a tough choice. Made me think of Wilfredo Gomez vs Lupe Pintor fight in 1982, when the two, one PR and the other Mexican, went back and forth hammering each other with as much skill and heart as you'll ever see until Gomez finally imposed his will in the 14th. Both went on to win other titles, but neither one was the same after that fight.
Mexican fighters have the greater tradition of ring machismo. On the other hand though, on a per capita basis, Puerto Rico produces the better fighters.
Mexico 106.000.000 population
Puerto Rico just under 4 million