As for me .... both are so useful to show a variety of speaking tones So i recommend you to mix it The example under shows the need of using both forms together 1. Only one form ( in a situation speaking to someone ) 집에 도착했다. 어제 근데 엄마가 없었다 어디간지는 모르겠지만.아무튼 그래서 엄마한테 문자로 이렇게 보냈다 나 진짜 이거이거가 먹고 싶다고 근데 엄청 화내면서 라면 먹으라고 했다.... 다~ 다~ 다~ you may feel the feeling tick tick tick which sounds like tough and cutting the groove 집에 도착했어. 어제 근데 엄마가 없었어 어디간지는 모르겠지만.아무튼 그래서 엄마한테 문자로 이렇게 보냈어 나 진짜 이거이거가 먹고 싶다고 근데 엄청 화내면서 라면 먹으라고 했어.... 어어어어 ~~ loose sounds ... i dont even know where the speaker is emphasising on or from what points the speaker was diapointed at exactly ... 집에 도착했다. 어제 근데 엄마가 없었어 어디간지는 모르겠지만.아무튼 그래서 엄마한테 문자로 이렇게 보냈다 나 진짜 이거이거가 먹고 싶다고 근데 엄청 화내면서 라면 먹으라고 했어.... By mixing them up, the groove of the speaking has been improved like ---다 ~~~~어 ----다 ~~~ 어 Of course, with two forms, it's not possible to express more strength or something like that But it's been more rhythmical and sounded comfortable. I hope it is helpful :)

Casual speech between friends uses both -아/어 and the plain -다 ending as you mentioned, but speaking entirely in the -다 ending is almost impossible because certain types of sentences require the -아/어 ending. Men tend to use the plain form more than women, but for both sexes the -아/어 ending occurs more overall. The -다 ending is similar to -아/어 in terms of the treatment of the listener, but has a more detached feel than the close and smooth sounding -아/어 ending. So it is often used when you make an observation or exclamation about things that may not relate to the other person directly. It is like the speaker is speaking to both the other person and themselves. There are some other usages too, like when children boasts about something (우리는 내년에 더 큰 집으로 이사간다!). Examples: - 야, 날씨가 참 덥다. A general observation, not necessarily telling or asking the other person. - 앗, 찾았다! (after looking for something for a while). It is both an exclamation to oneself and letting the other person know. - 나, 간다 = 나, 갈게. "나, 갈게" is more common, but some men may say 간다 more too.