[RPG] Experience and levels in Pathfinder

The systems have very little to do with one another. They are both trad games (as opposed to indie) and are both printed on paper. That's it. You won't be porting anything crunch-based from one game to the other (you can crib plots and characters, just not the stats).

Combat

Pathfinder has a complex D&D/d20-derived combat system with hit-avoiding armor and hit points that go up with level; everything scales sharply with level and there's huge disparity between high and low level PCs. Despite save-or-die stuff, lethality generally goes down with level because you have so many hit points.

Runequest has a skill-based combat system related to BRP, with damage-soaking armor and hit points that don't go up with level; lethality remains, especially if caught at a disadvantage, as characters progress. You never become a superhero like you do at Pathfinder level 10.

Magic

Pathfinder uses the traditional D&D "Vancian" memorize specific spells and cast them system. There's arcane/divine which don't differ all that much.

Runequest uses a spell point system and has spirit, divine, ritual, and sorcery, all of which differ.

Goals

Runequest is always going to be grittier and therefore fits investigation and low fantasy well. Too much combat will get you killed.

Pathfinder is always going to be more amped up - there are variants like E6 (never progress past 6th level!) that try to achieve the same thing but it's tuned as more of a mid-high fantasy game where heroes are low-grade superheroes. You don't have to play it as a combat focused game (I don't) but it has support for that.

Sure you can, as long as you meet the prestige class’s prerequisites. The Commoner class does not have any special rules about its advancement, it’s just a particularly weak base class.

It’s actually the easiest way to enter the Survivor prestige class (Savage Species), which requires that one’s highest base save bonus be lower than one’s character level. In a similar case, the Adept is the easiest way to enter the Hexer prestige class (Masters of the Wild), since it requires lightning bolt as a divine spell, and neither clerics nor druids get it, nor is it found on any Domain (amazingly).

I would comment that a common way to handle games where you start as Commoners is to have you “trade in” Commoner levels for PC-class levels, to avoid you taking pretty serious hits to your abilities relative to your nominal ECL. This is a non-issue if your DM knows how to compensate, but it might be worth mentioning as an idea to your DM.