I believe I have the Spintec RV muffler- though I'd have to check. I actually really like the sound it's just a bit too loud for my wife and two young'ins. The only time it gets really bothersome is when I'm trailering, which I do a fair amount. The truck had a Flowmaster 50 setup when I bought it, so I've never really experienced the stocker. I hope it's a good deal quieter than both the Spintec (and certainly the Flowmaster).
I replaced my stocker cause I couldn't even hear it in the cab. Since I like to control the gear changes, I had be watching the tach. Not good.
Make sure you get the resonator also if quiet is your goal. Though I ave to admit, I don't know how much difference it makes.