Best mufflers for added power?

I want to add a new muffler on my car (And possibly a cat back system later on) and need help choosing one. I was thinking a glasspack, but they're cheap and I've never used one. My mechanic offered me a nice muffler for $100, and said they're usually about $200, I didn't catch the name, but it was bigger than the glass pack, and had an insert to quiet it when needed. It was also smoked chrome rather than whatever the glasspack is made of. Has anyone used a glasspack before? Do they do much? My mechanic drags and does custom exhaust fabrications so I trust his opinions, but he didn't say much about the glasspacks. Are they any good? Do they add any power? They're cheap enough that if I don't like it I could replace it, but I'd rather not go through the trouble. Also, I understand a muffler alone won't make my car a rocket, but I'd like a better tone, and maybe a little extra power.