For me:
Pretty - correct
More pretty - correct
Prettier - correct
More prettier - incorrect
Much prettier - correct
Much more pretty - correct
Much more prettier - incorrect
I really hope there is a rule. Rules are good. I'm very much of the disposition that a language should be defined by how it is used, however. A rule, at least, would let me say that how it is used is correct or not, rather than this 'could be' that exists just now.
If there is no real rule, then we will just continue going round in circles, and 'agree to disagree' will prevail.
What I learned from back in junior high school English class was that as a general rule, "-er" ending is used with the words that are less than 6 letters long while "more" is used if they are longer than 6 letters. For instance, "beautiful" should be more beautiful while "tall" should be taller. Some words that are 6 letters long, such as strict, can go both ways as an except to the rule. The rule doesn't apply all the time but most of the time it works.