As far as muffler alone, you won't expect power gains you can feel. Mufflers are more about the sound you want. Borla sounds awesome but for the price meh. Magnaflow sounds good and doesn't rape your wallet. Flow master, meh. Are you looking to do just mufflers?catback? Or full exhaust?
Most people on the forum like to go with magnaflow. Solid product, good sound and quality and middle of the rode pricing FYi.