So you are trying to get shots while you fly? I suggest not a new camera...but a friend to come with to camera operate.
The difficulty with hat or glass cameras is that they will lack the telephoto required to see the plane when it gets some distance away. Video features in many of the point and shoots (Nikon, Canon, Pansonic) are very very good and even the smaller point and shoots have huge zooming capabilities (12x-20x zoom).
If you are really stuck to being the pilot and camera operator at the same time, you could try any camera that has a wide enough filed of view to see your entire flight area. Mounting the camera on something stable (tripod, table, ground) would be advanageous to a hat or glass mount because it will be much more stable.
If you must head mount it, you can try the GoPro or Countour helmet cams. I'm just not convinced it will provide the shot you want. For example, check the FT video of the glider tow plane. With the camera mounted on the glider, the tow plane looks like a spec in the shot, yet it is only 40 or 50 feet away.
Seriously, someone who can shoot for you may be the best solution.