I consider both necessary for catching the action. I have fitted a standard gopro pivot arm on a couple omer alien masks by drilling a hole and adding a bolt and nut. If a head strap is used, it should have an extra attachment to prevent loss of camera and alignment is very difficult. With a hood on, you might not even notice it gone. Lose a mask and you will notice it and maybe even catch it... Been there, done that.
Mask mount:
-shaky footage (unless very focused on slow head movements)
-flooding of mask (when hitting obstacles or during quick ascent)
-record on or off (difficult to check, I have record on always +extra battery pack)
-snorkeling on surface (waves splashing and shots on the surface not always visible)
-alignment (make sure lens of camera is parallel with mask lens)
+quick reactions/tracking
+you see what the camera sees
+first person feel
+film your buddy (without pointing a gun)
+film the unexpected moment
The bad stuff can the make mask mount completely useless, but when successful I think the reward is in the quality and the first person feel of the footage. I use high FPS setting on the mask because there's more action and movement. Also a non wide-screen format is good (for ex 1440p) because then you can fix some vertical misalignment during editing.
Speargun mount:
+stability (cannot be turned and shaken around like a head, the most important factor)
+camera button usability (easy access to buttons, and check the mode and recording state)
+shallow water filming (no waves splashing)
+sees what you shoot (good feedback for aiming, recoil and trigger technique correction)
-recoil (camera can flip forward due to recoil, even when properly tightened. a modified, shaped washer can prevent this)
-aiming (camera should be mounted to the side with an offset to allow aiming)
-loading (camera should not be in the way or get bent when loading speargun)
-tangles (reel line, weeds, kelp)
-slow tracking (never get footage of the shark that sneaked behind your back)
-drag, weight (affects reaction time, fatigue and buoyancy when handling the gun)
-how to get one (buy or diy, how make it fit all guns?)
-lose gun and camera (no show for the grandkids)
Speargun is perfect for darker conditions and photos because of the stability. I think the best ever filming mode for speargun is shooting video and a photo automatically every 5 seconds.
Here's a link to my channel. I try to film good footage, everything else I try to make bad...
http://www.youtube.com/salaseivastajat