Forrest is our best on-ball defender right now. He's solid and not as jumpy as Oni. Also, Forrest may be playable off the ball, as he's started hitting the corner 3.
Oni is mostly a defensive specialist whose defense is inconsistent. He's like a lesser version of Josh Okogie.
I would agree that Forrest is a better on ball defender, but I don't believe in his shot at all. I may not be a believer in Oni's either, but he is wayyyyy more proven as a shooter than Forrest. If you give Oni the same benefit of the doubt as Forrest as a shooter you might come away thinking Oni could be in the 3 point contest.
Without any belief in Forrest's shot I don't think he can make in on this roster. He could excel on another team that needs someone to run an offense and is OK with him dribbling the air out of the ball, but he won't ever get that opportunity here.