So I figured out the difference between the two by reading the manual for the Rocktron HUSH Super C. I may be incorrect in how I say this, but here are the differences.
Noise Suppressors - These are designed to eliminate certain frequencies from the guitar's signal, those causing noise, usually in the upper register. The threshold control is to pick the frequencies to be cancelled from the signal.
Noise Gates - These on the other hand are not quite what they sound like. What a noise gate does is completely cut off the guitar signal when it reaches a certain volume level. The threshold control on a gate is to set the volume level at which the signal is eliminated completely.
There you have it, I'm hoping this information will be useful to others as it was for me! What I learned also about the HUSH Super C is that the built-in noise gate has a special technology (forget the name) which cuts off the guitar's signal gradually, letting notes sustain completely (depending on how the controls are set) and not get cut off, as can happen with Noise Gates and Suppressors alike.