I have a
gopro hd - i didnt get it just for cycling but for watersports too so obviously being waterproof (rather than just resistant) was important after video quality. Since i've got it i have found the variety of mounts to be useful - i have mounts that are suitable for helmets, for handlebars/seatposts etc, a chest strap, a suction cup for surf boards and a wrist strap for scuba... this variety is where the gopro is strongest against the competition for me, however if it is only for use mounted on a helmet then i reckon the contour hd will be a bit better, certainly more streamlined. Video quality on them both are really good.
Video quality for uploaded videos is hard to compare though - in terms of userbase and popularity then youtube is far and away the best, however the videos are pretty compressed there and something that looks excellent prior to upload can lose alot of quality once on the site even when viewing in HD - with a site like vimeo the streamed videos are virtually the same quality as what it looks prior to upload.
I also have a muvi and while i found it pretty temperamental to use, the video quality was decent enough for the price and would be good enough for capturing incidents etc.