Flowmasters will NOT give you more power. Just sound. The idea of performance exhaust is to let more exhaust gases out. The flowmasters inverted design will never let as much out as easily as a flow-through muffler. Basically the glass pack will give you more power. Go with the glass pack or another brand of muffler (ie: Magnaflow). Keep in mind though that if your truck still has the stock exhaust system the benefits of just an aftermarket muffler are going to be minimal.