Of all the types of independent investigation that have been suggested, a special prosecutor is the most likely to disappear down rabbit holes—the least likely answer the questions that needed to be answered. A select committee of Congress or an independent commission of nonpartisan experts established by Congress can ask the broad question: What happened? A select committee or an independent commission can organize its inquiry according to priority, leaving the secondary and tertiary issues to the historians. A select committee or an independent commission is not barred from looking at events in earlier years statutes of limitations. A select committee or an independent commission seeks truth.
A special prosecutor, by contrast, seeks crimes. The criminal law is a heavy tool, and for that reason it is thickly encased in protections for accused persons. The most important protection from the point of view of the Trump-Russia matter is the rule of silence. A prosecutor investigating a crime can often discover non-criminal bad actions by the people he is investigating. If those bad actions do not amount to crimes, the prosecutor is supposed to look away.