I'll throw out another if you are on a tight budget: TachyonOPSHD. I have ran mine over a full season in modern and vintage karts, probably 75+ races, it has decent picture, really good audio, easy to operate, is about 2/3 the size of the Ion, and lighter yet.
If you want better picture go with the Gopro, Ion, or Replay XD1080. Those three also have more mounts available if you want to do things other than helmet camera. Replay and Ion don't use the protective clear cases that the Gopro and Tachyon use. Replay uses a lens protector on the aluminum camera body.