Which would you choose, Charisma or Wisdom as a stat source for deriving empathy (as a characteristic that defines character's empathy for other beings)? I'm having a dilemma here because both could fit. Thanks!
I'd recommend Wisdom, based on how that ability is described in the rules:
A character with high Wisdom has good judgment, empathy, and a general awareness of what’s going on.
Wisdom Measures: Awareness, intuition, insight (p. 8 Basic Rules)
Wisdom, measuring perception and insight (p. 57 Basic Rules)
A Wisdom check might reflect an effort to read body language, understand someone’s feelings, notice things about the environment, or care for an injured person. (p. 61 Basic Rules; Credit to @Christopher for pointing that out!)
Ability Checks for Wisdom (p. 58 Basic Rules)
Animal Handling, Insight, Medicine, Perception, Survival
As compared to Charisma, here's how that is described in the book:
Charisma Measures: Confidence, eloquence, leadership (p. 8)
A character with high Charisma exudes confidence, which is usually mixed with a graceful or intimidating presence. (p. 9)
Charisma, measuring force of personality (p. 57)
Ability Checks for Charisma (p. 58 Basic Rules)
Deception, Intimidation, Performance, Persuasion
As a DM you make the ruling. On the balance Wisdom looks like it wins over Charisma.
(PHB language is the same as in Basic Rules)
It seems like wisdom is appropriate for in-depth, rational understanding of what another person is feeling whereas charisma might be more appropriate for a spur of the moment intuitive understanding of the right way to respond to a person.
To put in another way wisdom is understanding them while charisma is knowing what to do to influence them. Though empathy implies benign intentions it is certainly possible to use the same understanding for more cynical ends. Obviously here there is a difference between not understanding another person's feelings and not caring about them. Here I think that the mechanics of reading another character are less important than how that information is acted upon.
For example a professional con artist may need a high level of understanding of people's feelings and motivations as well as the physical charisma to sell themselves.
Lastly, there is also an argument that the nuances of empathy are something where the role playing by the player is more important that a stats based character skill check.
In some contexts it may also be a case of observing body language etc. to determine a person's mood or state of mind through a skill check and then having the player make their own decision based on that information.
A good way to look at these 2 is to view them as attack and armor for emotions.
Charisma is the offensive attribute for emotions: if you want to achieve a result using your emotions, be it impress someone, convince someone or distract someone (i.e. Perform, Diplomacy or Bluff using the 3.5 terms), you attack them with your emotions and thus use your Charisma.
Wisdom is the defensive attribute for emotions: if you want to protect yourself against the expression of emotions by others, be it understand their emotions better (insight), understand why your animal companion shows an emotion (animal handling) or notice that they're not acting normal (Perception), you defend yourself against an emotional attack and thus use your Wisdom.
In this case, you're not expressing emotions, you're receiving them, so it's Wisdom.
I would say Empathy is partly both. The ability to understand the emotions of others would be Wisdom.
Charisma is the ability to express your own thoughts and feelings in a way to which people respond positively.