To some there is no difference and the terms are used interchangeably.
For others a slave holds more value than a submissive does as there is a deeper level of interaction between the Master and Mistress and their slave as oppose to that with a submissive. Some submissives don’t like to be called slaves even though they’ve signed a “contract” with their Master or Mistress despite the fact that in essence by signing that contract, technically, they became a slave.
The bottom line is that a submissive has more liberties than a slave does and some believe that a slave is more committed than a submissive.
For instance, where a submissive can decide and have some say in what is done to them, a slave has only the limitations imposed and agreed to by their Master or Mistress. This doesn’t mean that the slave has no rights, only that the slave has already negotiated those rights and trusts in his or her Master or Mistress to take them where they need to go and push them if necessary.
Also, a submissive tends to play around and interact for a limited time or only comes to play (dabble in what he or she wants then leave). The submissive is not necessarily interested in the more in-depth essence and connection of a D/s relationship as a slave is. This does not make them wrong nor “players” merely different in their mental and emotional connection to D/s. Think of it as the difference between a girlfriend or boyfriend (submissive) and wife or husband (slave).
I’ve found that someone with a slave mentality is very service oriented as opposed to a submissive. By service oriented I mean that they look to ensure that their Master or Mistress’ needs and comfort are taken care of; that their home looks nice; that if the Master or Mistress needs assistance in something they have it to ensure the Master’s day is pleasant.
Slaves tend to offer their assistance more readily then submissives. In most cases, a slave does not need to be told; they see the need and act on it. Of crucial significance is that slaves desire nothing in returned as the service they were able to provide gave them joy. In contrast, the submissive seeks some kind of compensation in the form of play, assurances, acolytes–but mostly play of some time.
There’s a different level of surrender associated with a slave as well. They are more open and willing to surrender all, whereas I find subs more guarded and apprehensive of their surrender.
Psychologically & emotionally, slaves are more enmeshed with their Masters and Mistresses than subs are. Again, the mentality and identity of the slave is vastly different to that of a sub.
I wouldn’t say one is better than the others (slave vs sub). Rather, I’d say it’s all in what the Master or Mistress desires and the level of commitment, connection, and most importantly surrender that is required or desired from the individual.
Please note: opinions vary even within the community and amongst slaves and submissives themselves. However, these are merely a few clinical observations and personal experiences combined.