"Le vi" vs. "Lo vi"
I've seen both used for "I saw him" but I was wondering if anyone had a reason as to why both are considered correct/one is considered correct/etc.
This is a phenomenon called "loismo" or "leismo" (depending upon which object pronoun you use). It's based on regional variations; in some places "lo vi" is more common, and in others "le vi" is preferred. Just pick one and stick with it.
Does it have to do with Spanish from Spain vs. Spanish from Latin America?
In some areas, lo is the pronoun for "it." You will find different words and idioms used in Latin American countries and in Spain. For example, in New Mexico a kind of pigeon Spanish is spoken. Here, the word for court is corte, but in Puerto Rico it is tribunal. The word for car is carro, but in Spain it's coche. The word for truck is trucke here but camion almost everywhere else, including Mexico. Stop signs in Mexico read ALTO. In Puerto Rico, they read PARE.