I do not think the Contour Roam's GPS is usable for anything other than time & location-tagging your video; maybe adding a speed or map display as well. If you want a GPS for real-time navigation, get one.

I have an older Contour (HD). I use the handbar mount with it, and (so far) have not had problems with it getting rained on. Mount is quite solid, although the mic seems to pick up a lot of mechanical noise from the bike that I never hear myself. (could just be lousy hearing). The headband mount works well for things like snorkeling masks (as does the waterproof case!) but won't attach well to a typical bike helmet. I haven't tried the adhesive helmet mount. I've seen pictures of Contours mounted on different spots of a bike frame as well.

The Go-Pro is larger than anything I want on me or my helmet when riding.