On a bolt gun you gain some quietness by running a front cap/exit hole close to your bullet size, but on an AR there is so much noise from the ejection port it doesn't really matter.
I'd get something 30 cal, 7" or less that is entirely or mostly titanium. Ounces matter when they are out on the end of your barrel. This isn't something you can re-sell easily so get the best thing you can afford because practically speaking you are stuck with it forever.
