After choosing Hermosa, I don't have the option to upgrade the Charm skill. Does choosing the other trainer allow you to upgrade Charm? Does anyone have a list of what skills each trainer can upgrade? Thanks.

Not all skills are available to upgrade over rank 5. Rank 5 Charm skill already allow you to charm 3 creatures up to level 37. There no point into providing you upgrades on the skill, as the highest level boss in game is just lvl34.

That makes sense, thanks. So both trainers offer the same skill upgrades then?

Yes, both trainers offer the same spells.