I had the RT66 on my car for a year before installing the Magnaflow. Honestly, I can barely tell the difference between the two a idle or while driving at low speed, they are almost identical. The RT66 may be a little louder, but the difference is really small. At highway speed, the Magna is definitely milder. Less drone, but less overall sound as well.
I liked the RT66, but it did have a drone that I had to shift around on the highway. Once I added headers, the drone became more annoying, so I went with the the Magnaflows. I thought the RT66 looked a little better on the car, the tips are nicer. The MidAmerica chart doesn't really match my real world experiences, I heard a very big difference in drone between the Corsa, Magnaflow, and RT66 on the highway, but they all rate a 1?