I think its all about your personal perception to me a slave have way less choice in what they have to do and whats done to them and i hope this was agreed on beforehand.As far as a sub they have more imput and choice and they do what they do willingly and freely. What can i say but thats my opinion and i am sticking to it. So in short its up to you to define it for yourself what works for me might not work for you. Well plus i do not personally like the term slave but again thats me