It feels as if Suisei is always forcing too much to sing. It can't be healthy to her throat.
https://www.youtube.com/watch?v=8xRJmEY7g_sWhen compared to Gura, Gura is almost talking, she will be able to sing forever:
https://www.youtube.com/watch?v=l5ww0939E3sAlso in my opinion Gura, sings better than Suisei. Just listen to the video above and the stuff Gura does with her voice.