Glass Pack vs Catback System?

I recently put a glass back on my truck and I feel that I might of lost some power and gas mileage so what is the differents between the glass pack and the cat back system.Do you think I would have better results if I change it to a cat back system I was think about the dyno super turbo exhaust. I have a 2000 ford ranger with the 3.0 v6 engine thanks