If you want to remember just one general rule, put the "more" before the noun, because there are some situations where "more" can only go first, even though both of your examples are fine. I think "more" after the noun requires a number in the noun phrase, but I'm not sure. Maybe someone else has more information on this.
Many more people like vanilla than chocolate. *Many people
There's no doubt about it, Guest, that can SOMETIMES occupy both positions. The kicker is that some collocations are simply unidiomatic.
What defines those differences; can't say at the moment. ****, might not be able to say given a month of Sundays of solid thinking. Such is the complex nature of language.