Object collision

With the new build out there are additional ways of making objects “no collision”, you can set them as ghosts, set the object type to No collision, or set the individual faces to no collision. Does anyone know the key differences between these, and which is better to use in which situation/which are cheaper on resources?

Ghost objects still obey physics in terms of falling, forces, etc, but don’t interact with other objects. However, a collision between a ghost and another object can be detected (even though it doesn’t change the motion of the ghost).

I guess “no collision” would indicate that an object doesn’t cause the collision actuator to fire… but like ghost otherwise?

I’m pretty sure setting the faces is a feature held over from previous versions. Not so sure about its usage.

speaking of collisions…is there a way to stop collisions between only 2 objects?