TLDR: Core Rules Set Enemy Difficulty to "weak" and Enemy Stat Adjustments to "normal" for mostly-accurate Pathfinder experience, in terms of encounter balance.
TLDR: Enemy Stat Adjustment only seems to influence Regeneration and Immunity bypass (no or reduced damage from non-magical weapons), and from what I can find in the code, only when choosing "somewhat easier" (half regen) or "much easier" (no regen) options. Any other option, such as "much tougher enemies" doesn't appear to actually be used anywhere I've found in the code. Comically enough, this even means choosing the "moderately easier" option is exactly the same as "much tougher" when it comes to this (which is all I can see it actually getting called for).
TLDR: Enemy Difficulty changes the modifier scores (but not the base values) of the main attributes, skills, AC, Attacks, and Saves, by -2, +0, +2, +4, for weak, normal, strengthened, insane options accordingly. I'll go into why below, but you might notice attacks and AC get double-dipped due to the way the math is handled. I'm not sure if that's intentional or not, but it results in a rather steep increase in certain key stat values.