The simplest answer between the difference between a submissive and a slave is summed up in one word: Negotiation.

Simply put, the submissive has the right to negotiate new rules for herself, or to alter rules within her relationship after she has enetered into the relationship.
The difference IS choice and in essence negotiation, most absolutely. Once you are a slave, your choices are made for you. The only choice a slave really has is to leave or stay. A submissive that is not a slave has many choices and because of that many things ARE up for discussion and negotiation. As a slave, you trust that your Master knows what is best for you and he makes the choices.

Though, I still differ on the level of commitment. I believe that a slave's level of commitment is much stronger. Just the fact that she must trust her Master with her life is reason enough for me to believe that. A submissive is someone that will allow someone to make choices, up to a point, but she has the last say. A slave truly has very little say if any. And, simply because of that, knowing that a submissive can redefine the rules and the relationship at any given moment, I believe that the level of commitment to the relationship is not as extreme as a slave's. I am NOT saying that a slave is not commited. I am saying it is a very different level of commitment that a slave has.