If you reach a plea agreement with the prosecutor, assuming the judge accepts the plea agreement, then you know what your sentence will be. If you go to trial and are found guilty, there will be a sentencing phase where the prosecutor will typically argue for a harsh sentence and your attorney should point out mitigating factors, reasons that you should receive a lesser sentence. Therefore, it is a matter of fact that you might receive a harsher sentence if you are found guilty, but you might not, it depends on the facts of your case and information about you.
Depending upon how the information is being presented by the prosecutor, it could be prosecutorial misconduct. If the prosecutor is merely pointing out that you could receive a longer sentence, that is one thing, But if the prosecutor is stating that if you exercise your constitutional right to make the state prove every element of its case beyond a reasonable doubt, then I, as a lawyer, would point that out to the one who decides the sentence, whether it be a judge or jury. In my opinion, that sort of threat abuses the constitutional system that we live under and due process of law that you are entitled to.
Good luck to you, and please remember to designate a best or helpful answer.
What you are asking cuts to the heart of why the plea bargain system works. More often than not, when a person is charged with a crime, they're technically guilty of that crime. Morally, whether they should be charged with a crime for the act in the first place is another story however.
Even when the state has an airtight case against a citizen, oftentimes an attorney can get a client a plea deal that results in a sentence more lenient than what you would get had you gone to trial. The reason for this is simple: the state doesn't want to take cases to trial. If a case goes to trial there's always the possibility that the state loses. Therefore, a plea bargain is basically the state living by the saying that "One in the hand is worth two in the bush."
So what the prosecutor is offering isn't what he believes a just punishment necessarily is, but rather getting as close to what he feels in an appropriate sentence while minimizing his risk of losing the case at trial. It then follows that if the prosecutor must prove his case at trial, he's then going to seek what he feels is the appropriate sentence -- something more severe than what he's offering as a plea bargain.