BTW Ben, you might know this, but noise suppressors work by 'closing the door' so no signal (or hiss) can pass through unless a threshold is passed at which the door opens a bit, depending on the sensitivity. Thus depending on the level of hiss you can balance it just right to close just after the sound becomes lost to hiss without cutting off any sustain, but it depends on how loud the hiss is. Chances are if it is noticeable enough to annoy you at the moment then a noise gate might not work. On extremely low level hiss I don't like gates because I notice it far more than without....
Cheers
ped