I own both the octane and obsidian.
The obisidian may sound slightly less louder than the octane (to me, it does sound slightly less louder). To some, they can't tell the difference, particularly if they're not doing the shooting.
If you can get the octane for about $150 - $200 less than the obsidian, I'd go for the octane.
Very little to no blowback on either can on either a HK45 or 220 from my own experience.
The obsidian does allow for modularity over the octane though.
If price is of no concern, I'd give the nod to the obsidian.