Haven't tried the res back system, but I am happy with the Dynomax muffler. I have the 3" in/out version of the muffler that comes with that kit. Nice sound, little quiter than a straight pipe and no drone.
If you go for system, spend a couple of extra bucks and get a 3" to 4" adapter and a res length of 4" aluminized pipe and get rid of the resonator.