Note that the later design has a raised boss for the spark plug, a wider squish zone that is evenly domed, and a much thicker material section at the back of the head.
There are also piston differences. The '84-85 pistons are dished, whereas the '86-87 pistons (and all service parts for '84-87) are domed slightly. Since they are service-interchangable, the head appears to be interchangable as well. However, if you start shaving heads this will be something to watch.
The '86-87 piston and head would appear to be the better performing combo. The head design, being evenly domed, should allow more material removal to increase compression. The earlier '84-85 design is flatter and actually has reliefs towards the circumference. These would, in my mind, negatively affect the combustion flame growth. As many members have noted, the '86 and '87 Spree are faster "out of the box," with top speeds around 35 MPH. The '84-85 models rarely seem to climb out of the 30 MPH range.
Have fun.