In theory grand juries are supposed to be a check against government overreach by requiring the jury to settle on the charges that will be brought rather than the prosecutor. And in some cases that in fact does work out that way. But quite often the prosecutors are quite skilled in convincing the jury to indict on the charges sought, so it ends up not presenting much of a barrier for the prosecutor. You may have heard the expression that a prosecutor can get a grand jury to indict a ham sandwhich. That gives a sense of just how often the prosectors get what they seek. In the federal system the Constitution requires grand juries for most felony cases. Often the grand jury ends up being a help for the government, the chief benefit being that the grand jury gives the prosecutors additional tools to use in investigating the case and bringing pressure to bear on potential defendants.
But in terms of bias, there are no studies I know of that clearly show grand juries are more or less biased than the prosecutor, and I suspect that there is likely no real consistent pattern in that regard either way.

