Definitely a difference between different types of obsidian. I think the differences appear because of the various cooling times of the original lava flow. The minerals within the obsidian causing the banding, layering, and colors affects the properties as well.
The smokey-clear, double-flow, and triple flow obsidian is a lot like modern bottle glass. I think it's the best and most durable but I have a hard time knapping small pieces from it. It's great for the larger pieces, though. Sometimes it has layers in it, so you have to watch out. The layers will stop flakes from traveling if you are unlucky enough to catch the layer just right.
Mahogany obsidian is the opposite. I like it best for the small arrow points. It also seems to have the least amount of internal flaws.
The "sheen" obsidians are very brittle and I don't like knapping that stuff. It make really nice points when it works, though.
I just got a chance to work true "Black Butter Dacite" for the first time at a knap-in last weekend. It's amazing stuff. It's jet black, not translucent, and knaps a lot like Mahogany obsidian, but not as brittle.
There is a blurry line between the different varieties and you really have to test each piece to see how it knaps. I got a bunch of obsidian samples recently and even though it might look the same, it may not knap the same.
