When I got my GoPro, I thought about different mounts for a bit. In the end, I decided to go with the GoPro mask mount over a gun mount for several reasons.
1. It can always be with me. Even if I don't bring the gun on a dive, or just wanted to jump in to film a sea turtle or something, it would be ready.
2. It records where I look. There are times when I'll want to get some video of a buddy stalking and taking aim on a fish. I'd rather not do that with the gun pointed at my buddy.
3. The shot is usually disrupted by the recoil of the gun when it is fired. You could have the shot lined up perfectly, but as soon as you pull the trigger, the gun shakes, so the video of the moment of impact is not viewable.
4. Better maneuverability. There are places that you may want to video where your gun can't quite get the right angle.
I ended up going with the XS Scuba GoMask. I liked it better than the Octomask, and has a stronger mounting bracket (metal vs. plastic).