I play really tight to my marker, and full blocks just don't work for me. The beavertail is always hitting me in the mask, and if I remove it, then I get smacked in the face with the backblock. For this reason I prefer halfblocks. But that is just me.
One thing good about an S6 is that the lower tube internals are not exposed to dirt, etc through the body slot (as Stanchy said). I am very anal about keeping my markers clean, so that is an advantage over the 6.5 in my opinion. Still, I would always choose the 6.5