理性 and 理智, these two words can be used as noun and adjective to describe someone who's smart. But We use 理性 to describe someone's personality. say 他是个理性的人. It's paired with 他是个感性的人(he is a sensitive man). However 理智, we use it in a more particular situation, e.g. 当Jim被Sam激怒时，他没有失去理智和Sam打架(when Jim was provoked by Sam, he didn't lose his control to fight with him). We say Jim dealt with the situation in a very smart way. In short, use 理性 to describe something abstract like a person's personality and 理智 when people are smart enough to deal with a particular situation.