I just did a lot of research on this and concluded pretty much what the posts in this thread have already shown. Once you decide to buy a quality brand, it's pretty much personal preference. The audiophiles will tell you this one drones more or that one is a bit throatier, but you won't find much consensus. Youre also going to run into a huge debate over added horsepower. Again, there's a wide swing in thought on this. What I was able to decipher is that the HP gain, if any, might be in the 2-5 range, and you might be gaining those HP just from the cool factor of having a specific brand.
I ended up with a Magnaflo, new cat and muffler, turned down right before the axle. Just installed yesterday. Sounds great compared to the cherry bomb glass pack that was on it. It's a little throaty on takeoff, but won't piss off the neighbors, doesn't seem to drone on the highway, and installed with cat, cost me under $300. At least half of that was the cat.