Hear me out
Both were undoubtedly useful. If we're talking trails ? Chiaki mogs. But overall ? Let me explain
Kazuichi fixed the elevator in chapter 4. If he didn't we wouldn't have gotten to the other side. So we wouldn't be able to work together. And we could barely communicate. So basically kazuichi saved all of our lives. No one besides him could fix. When it comes to chiaki tho. She also helped. By not letting hajime go to the final dead room. So she also saved all of us in extent. But the thing is. Anyone could have stopped hajime. But ONLY KAZUICHI could have fixed the elevator
In chapter 1 kazuichi made sure peko wasn't in the circuit room or wtv. And chaiki also helped with the floorboards thing with Gundam. Also giving fuyuhiko an alibi.
Aside from chaiki's sacrifice ? She barely did shit. Unlike kazuichi here who ACTUALLY saved all of us AGAIN in chapter 3. How did hajime know >!mikan!< was the culprit ? That's right. The camera angle. And who made the camera on the communication device ? KAZUICHI SODA. He saved us TWICE. Just like chiaki.
Also he discovered Mahiru's dead body. Made minimaru. Gave fuyuhiko something to read while he was injured
Overall ? Kazuichi in underappreciated and overhated