The real difference is that when you get picked up for a DUI, you go running to your "attorney", when a drunk driver kills your daughter, you go running to the prosecutor.
And the difference between a trial and a grand jury, is the prosecutor controlls the grand jury - no one questions him/her.
From Wikipedia:
“Criticism
The most persistent criticism of grand juries is that jurors are not a representative sampling of the community, and are not qualified for jury service, in that they do not possess a satisfactory ability to ask pertinent questions, or sufficient understanding of local government and the concept of due process.[21] Unlike potential jurors in regular trials, grand jurors are not screened for bias or other improper factors. They are rarely read any instruction on the law, as this is not a requirement; their job is only to judge on what the prosecutor produced. The prosecutor drafts the charges and decides which witnesses to call.[9] The prosecutor is not obliged to present evidence in favor of those being investigated. Grand jury witnesses have no right to have a lawyer or family in the room, and can be charged with holding the court in contempt (punishable with incarceration for the remaining term of the grand jury) if they refuse to appear before the jury[22] and all evidence is presented by a prosecutor in a cloak of secrecy, as the prosecutor, grand jurors, and the grand jury stenographer are prohibited from disclosing what happened before the grand jury, unless ordered to do so in a judicial proceeding.[9]
After a grand jury was commissioned to investigate whistleblowers organization WikiLeaks, grand juries have been accused of being used as an intimidation and persecution mechanism against whistleblowers and anti-war activists.[23]”
Says it all.