The other difference is of course the SLP uses angel hair fiberglass in their mufflers, which means if you are looking for something crazy loud...go with the SLP, because over time its going to get even louder......
The Magnapacks are going to stay the same volume over time- because of the Italian Rock Wool....
Based on the dynoes I have seen, the Magnapacks seem to make a little more power too.....
Muscle Mustangs and Fast Fords just did an article, and with a supercharged GT, with some bolt ons, the GT only gained 11rwhp and 5rwtq with the SLP
Ive seen stock Mustangs gain that and more with the Magnapack sytem.