Well, in all the talk I have now heard about the difference between the two, I have heard one persuasive remark, which is this: Prosecutors who have handled this sort of case know perfectly well that the minute someone is identified as a "target" of their investigations, that person clams up, and from that point on there is zero hope of any sort of cooperation or willing testimony from them. Mueller apparently still hopes to induce the egomaniac Trump to sit down for a face to face interview, which very likely will be the biggest mistake Trump ever made, and Mueller knows that if he lays all his cards on the table, Trump will run for the hills- perhaps the hills just outside of Moscow, if any exist. So he'll take what he can get now, and then we'll see what he really has, after Trump has a good chance to perjure himself but good.
Wishful thinking on my part? Maybe, because I am not a lawyer, let alone a federal prosecutor, but there is a certain amount of sense in this explanation.
Update: From the Washington Post this morning:
"The president has privately expressed relief at the description of his legal status, which has increased his determination to agree to a special counsel interview, the people said."
This Trump guy is as easy to manipulate as it is to trick your dog into going for a walk. World's greatest negotiator, indeed.