One more thing. Roughly speaking, 使う looks like an everyday dress, and 遣う the best one. I think people like to choose 'an everyday dress' in ordinary conversation.

I've looked those words up on several dictionaries. Firstly, 使う and 遣う have the same meaning for the most part, but there is a difference. 使う is used for the word つかう in general because it is easier to write and read. 遣う is used in specific situations: to say 'use your head a lot', to say 'use money', and a fancy way of saying 'use tricks or skills very well'. Secondly, a few dictionaries show only 気を遣う for きをつかう, and some say 気遣う is only used for きづかう. But others don't refer to these differences. So, I think 気を遣う is more correct, but 気を使う is also OK. I think 気を使う is just a substitute for 気を遣う, but sometimes some people might choose to use 気を遣う to emphasis something because there is actually a small difference between 使う and 遣う.

気を遣う＝相手に対して思いやりがある気の遣い方（親友に対して） 気を使う＝相手の機嫌を取らなければならない場合の気の使い方（上司とか） 遣う＝some kind of will exist (from bottom of heart)　　　 使う＝force to do (polite in diplomatic way) こんな感じだと思います。