Apex'I is the quiter of the two, from my experiences, but its a very small difference. The RS-R is just a tad bit louder but it does have a stealthy design. The Apex'I canister is kinda large and can be seen from afar but for $10 @ any service station u can get the hangers bent upwards and that eliminates that problem. I quite like the Apex'I as its barely audible during highway cruising but when u stomp on the gas it utters a low growl. Combine that growl with an intake and its a kick-ass sound combo. Hope that helps