The responsibility for deciding on this issue rests with the defendant and his or her lawyer.  Only they (and ultimately the defendant) can decide if the defendant should testify.

The reason for this lies in the Fifth Amendment.  According to this amendment, defendants cannot be made to testify against themselves in criminal cases.  Because of this, the prosecution cannot force a witness to testify.  The defense has to decide whether the defendant will testify.  They must decide this based on how well the defense thinks the jury would perceive the defendant and on things like how well they think the defendant would stand up to cross-examination by the prosecution.

But these are tactical decisions.  Legally speaking, only the defendant (with advice from defense lawyers) can decide whether the defendant will take the stand.