Do you plan to use the can almost all the time or only occasionally? Using it only occasionally on a short-barreled rifle will mean you need to make some decisions before you buy the SBR on whether you want to optimize it for unsuppressed or suppressed use. The shorter your barrel, the more narrow your "operating window" is and the more dramatic effect putting a suppresor on the barrel will have on that window.
In general an AR suppressor makes the gun quieter by suppressing the muzzle crack; but you still have the mini-supersonic shock wave that accompanies the bullet. Because a longer barrel burns the powder more thoroughly and gives more volume for combustion gases, it will generally be slightly quieter than a shorter barrel with the same suppressor.
The flip side to a suppressor is it adds weight, length and heat. The suppressor will heat up much, much faster than the barrel. Nomex gloves would probably be a worthwhile investment for a sem-auto SBR and suppressor combo.
My review of my Ops Inc. 16th model suppressor on an AR:
http://thefiringline.com/forums/show...hlight=ops+inc