if you can afford it, get the sting bed and you will be glad you did.
one place i used to go to had the black poly and ended up replacing just the bed alone with a string bed... it gave a good comparison of the two in that everything else had remained constant. They are a lot more responsive,being more rigid, they use a greater portion of the springs compared to the black poly. The plus with the black poly is that its less harsh on your feet
what size tramp and how big are the springs? if the springs are too small (8 or 9 inch) its possible to blow through the springs real quick with a string bed... it will make it feel like its bottoming out.