I use both, I have the ContourGPS, friend has the GoPro, both have advantages and disadvantages. Main advantage the GoPro has is you can mount it on your chest.
Contour has GPS, so you can add your speed, map and things to your video, also edit the settings via your mobile phone, and preview the live feed. This does eat batteries, but thats fine, as they are a Nokia battery, about £8 each.
GoPro you can chest mount, I love the view! If you want chest mount, then GoPro it is..
If you do go for a Contour, go for Contour+, 170° viewing angle and has the GPS and bluetooth. I have used mine in the rain its been fine.
[video]http://www.youtube.com/watch?v=iYaJ-tiRJ2k[/video]
Here’s my test footage of my GPS/speed/track overlay, with the right software, you can create custom dials/maps etc. I use DashWare & Premiere Pro.