I have a Diamond Eye Quiet Tone 4" and I really like it. A lot of the bigger exhausts tend to drone, and after a couple hours driving, it no longer sounds "nice", it drives you nuts!
You might want to spend some time on YouTube listening to the different exhausts, because at the end of the day, it is personal preference based on sound. Deleting the catalytic converter can pick you up 1-2 MPG, but don't expect any fuel mileage gains other than that over the stock exhaust. All the aftermarket units flow so well they are going to get the same mileage.