What type of evidence, direct or circumstantial, is more useful to prosecutors in trying to prove a case?
Ideally, a prosecutor would like to have both kinds of evidence because both types of evidence have their flaws. It is best to have both sorts of evidence so that their combined weight can prove the case beyond a reasonable doubt. There are arguments to be made for the superiority of each kind of evidence.
On the one hand, it seems clear that direct evidence is the best evidence. Direct evidence is often given by witnesses who have seen the crime occur. It is very powerful to be able to put a witness on the stand who will testify that they saw (for example) one person shoot another person. Their evidence does not need to be interpreted; it stands on its own.
However, that kind of evidence can have its value destroyed if the witness can be made to look unreliable. The prosecutor has to hope that the jury will believe the witness even after the defense gets done with cross-examination. This can be a real danger.
Therefore, it can be argued that circumstantial evidence is better. Let us say a prosecutor has evidence that a certain gun was used to shoot Person B. Let us then say there is evidence that Person A owned that gun. Let us further say that there is evidence that Person A had reason to be angry at Person B. Let us finally say that there is evidence that Person A was in the area where the shooting occurred at roughly the time that it occurred. Let us say that none of this relies on eyewitness testimony but rather on things like receipts, legal documents, and surveillance cameras. All of this evidence together could be much more convincing than a poor witness who saw the shooting occur.
Thus, either kind of evidence can be better than the other. It all depends on the particular details of the case and the evidence. It is much better to have both kinds of evidence than to just have one.