Jenny,
The sleep bras are more like a sports bra...usually a sort of racerback style and the cup stretches down off the breast without any sort of clip or hook.
The softcup (as opposed to underwire, which is not advised for nursing even though they make them), usually has a clip or hook on the strap that holds the cup in place, and under the cup there is some sort of full or partial frame around the breast.
I love my Medela Softcups, but only the seamless kind. They are totally worth the money. I used them for more than a year with my first daughter and am using the same three for going on 15 months with my second and they are still like new.
Nicki :-)