For me, headaches (migraine and tension type) are present constantly whereas head pressure typically occurs only after over-exertion or over-stimulation. In my case, it feels like there is pressure in my head that builds following these situations. Sometimes it manifests as an uncomfortable feeling as I am talking, but may actually be intense enough to cause pain. Alternately, I encounter head pressure when laying down, dipping my head, etc.; I think this is more or less normal, but since my concussion the pressure I feel when doing these things is much "higher".

I believe the pressure is associated with blood flow; blood travels to and from the head differentially such that "pressure" builds. The pressure effect for me is fairly global across my scalp. To be honest, though, I'm not sure if head pressure and headaches are exclusive events. I suspect they are not, though like everything else associated with PCS it is likely very complicated.

Mark may be better able to contrast the two.