Sejeong was in The Uncanny Counter, which was a pretty popular drama both in Korea(average rating of 7%) and overseas. She was also on Busted!(a variety show with 3 seasons) for a few episodes along with Sehun, Yoo Jaesuk etc. Arguably, we would need to wait for her future acting endavors, but she's doing pretty well so far.
She's a part of the main cast in Busted, not just "for a few episodes".
It's 1A and 1B between Chungha and Sejeong imo.
Sejeong has had more activities in variety and acting, and she's only just getting started in solo music.
Chungha mainly focuses on music, and some variety. Chungha is clearly more recognised in music