I have the turbo style mufflers on my 4x4 with headers and glasspacks on my 72 with stock iron manifolds. Both trucks have 2.5" pipes. The 4x4 has a bad tendancy to "resonate" at idle inside the cab. It can be annoying. Personally I prefer the glasspacks. They make my nipples hard!
Here's a bit of info that alot of people don't get though, the longer the glasspack the deeper the sound. If you use a 12" glasspack you're just going to get loud and poppy like echo chambers (which I hate). If you run a long glasspack like mine (30") then you get that deep mellow sound. You don't get backpressure from a glasspack, they're a straight through design as long as the core of the glasspack matches your pipes. Mine have true 2.5" cores. I'll give you a video of the glasspacks to listen to however, the exhaust sounds alot louder in the video than it really is.