Most important,...
You want a lens 120' wide angle or more...(this will smooth the shots and prevent underwater effect from vibration)...
You want a good mount... (on your body is best... this will smooth the shots, using your body's natural shock absorption).... hard mounting to the bike is never as good....
Somewhere between eyes and chest level works best... (shows a bit of tank and bars,.. along with road and sky)...
Beyond that,.. you pay for resolution,.. battery life,... and durability...
and fancy stuff ( wifi, bluetooth, etc)...
I like a rear lcd screen, with menu's... (they are more descriptive, and easy to use)...
Go-pro is the pricey standard.... sj1000, 2000, 4000, are cheaper with some better features, but less selection in mounts and accessories...
Many others out there too...
I have an old low res, GoPro... and a newer sj1000 (I haven't really used yet, beyond a few tests)...
Visual/audio quality is easy... good mounting, and ease of use, are the things that make the difference...
Best luck...
Show me some fun rides...