The FlowFX and American Thunder Kits are more or less similar when it comes to durability. They're both 409 stainless steel systems and will hold up well in most climates and applications. The FlowFX is a single-exit kit with a more traditional straight-through muffler, which is why it's less expensive than the American Thunder System with dual exits and a Super 40 muffler. In general, you won't see much performance from a cat-back alone, and cat-back choices should be made based on aesthetics and sound more than horsepower and torque gains.