One collects the oil blow by (Crawford)the other one recirculates it to the intake/ back to the oil pan (GrimmSpeed)
GS aos have been know to allow oil to slosh back up into the intake tract on hard right turns.
Crawford: awesome for track does what its supposed to. You have to empty it periodically though.
GS: set it and forget it don't use it on the track, unless you like burning oil.
I personally use the GS and have no problems with it, but I also do not track the car. It has definitely stopped all/most of the blow by making it to the tmic, and I only burn an 8th~16th of a qt per oil change (3750 mi)