The coil futon with the built in hinge is the upgrade you didn't you needed.  In bed form it feels the right amount of stiff/soft and you don't feel any of the cross slats of the futon frame.  In seating mode it feels stable and springy, the hinge is seamless and fits my old futon perfect.  It takes a little more effort to get it into position that my old floppy mattress but worth the effort.