Difference between 決{き}める and 決{けっ}する?

Question

I'm reading a text that has the word 決{けっ}する, and so far I've only seen 決{き}める to mean "decide". I've searched a few dictionaries and couldn't find the difference between the two. Is there a difference or nuance I'm missing?

Answer 1

決める

• a basic word that everyone knows

• basically a weak and casual word like "make a choice", but can also represent stronger decisions (like "make up one's mind"); for authoritative decisions we usually choose other words

  ピカチュウ！　君に決めた！
  I choose you! Pikachu!

• transitive; the intransitive counterpart is 決まる "be decided"

決する

• a big, bookish word that wouldn't be uttered by a child
• means to make a conclusive settlement (usually, of yes-and-no or either-or) through the decision
• transitive or intransitive

  運命を決する (it) determines the fate
  運命が決する the fate will be determined

Answer 2

決する mainly appears in some set phrases such as 勝負を決する, 雌雄を決する, 意を決する. 決する can be used in legal or judicatory contexts, too, although it sounds more stiff than 決める. けっして with a negative expression means "never", but it's probably better to regard this as a distinct adverb.

Except for these, using 決する instead of 決める would sound fairly literary or even funny. Don't say something like 明日どこに遊びに行くか決する, for instance.