The difference, according to the lawyers who served as my professors, is that one is considered by the courts to show intent to contract, and that the other is considered by the courts to be negotiation.

That said, I'm just repeating what a lawyer taught me about how courts determine intent to contract. I'm not a judge or a lawyer. I'm an entrepreneur who took some business classes.

edit: I just sent an old professor an email with the question, and a prompt to try to help him remember who I am. If he responds, I'll share the response no matter what it says.