I prefer helmet mounting. Most problems I've encountered in traffic were recorded better from the helmet mount because it recorded where I looked, including brush by passes.

Two disadvantages to helmet mounting:
1. The videos have relatively little vibration but are very swimmy. My head bobbles a lot because of an old C2 neck injury.

2. Battery life. My Ion Speed Pro runs 90 minutes per charge. I need to rig up a small external battery for longer rides. This is easier to do with a handlebar mounted camera.