Rock Wool. This was one of the more interesting choices we made. I'm not sure how I stumbled onto it but I ran into rock wool and did some research.
Rock wool is made of slag and volcanic rock. They heat it up to ridiculous temperatures and then whip it like cotton candy. However, instead of it being light and stringy like fiberglass insulation it ends up being thick mats.
What got my attention with rock wool was the sound deadening properties because of the cross fiber (thick mat) and the weight of it (one of those packages weighs 60lbs).
Killing sound is important living next to train tracks.
Another thing about the cross fiber business, it is thick and harder to burrow through or build nests in - making it less rodent friendly.
The final and really cool thing is how fire resistant it is. My parents house burnt down in the 90's and I've been a little bit leary of fire since.
For all those benefits it is not much more expensive than fiberglass. It's also much more pleasant to work with (but still itchy if you're really sweating). Oh yeah and it's "green". Rock wool is made from slag - essentially a waste product that would go into landfills.
Check out the video below. If you don't have the patience, here's how it ends. SPOILER ALERT: The fiberglass melts in no time, they get bored and shut off the camera before the Roxul lets flames through.