Suisei is technically a much better singer. It's not even close, Marine can barely hit her notes.

However, I still prefer Marine's songs because they try to be unique and experimental. It has a lot more charm.
Suisei's songs on the other hand feel like slop. Good slop, but slop nonetheless.

In terms of streaming, Marine is also much more charismatic and much more witty, whereas Suisei is a one-trick-pony.