I have heard neither, but Holdeshot does offer some sort of noise reducing baffle that they say will cost 1.5 hp, if you think it is too loud.
Ideally, you'd find someone who has one. For me, I live about 45 minutes from Holeshot, though I doubt he'd have a slipon'd equipped C14 for me to listen to!
As for stainless vs black, that is subjective, but considering how the C14 has patches of black all over it (under fairing area, plastic pieces, etc) I can see it looking better with a black can. With silver, you'd see bike and pipe as there is nothing shiney to go with the stainless pipe.
Just an opinion...