I would recommend getting something in either MiniDV orDigital8. Digital8 is going out of fashion, but is less expensive to buy (and replace), and the tapes are also less expensive. Also, you can use Hi-8 tapes in a pinch. MiniDV cameras are usually a bit smaller than Digital8, as are the tapes. The Mini DVD cameras seem convenient, but if you intend to do any editing, they typically convert the video to MPG before burning to disk. This hurts the video quality in the end. If memory serves me correctly, the hard drive recorders also record in MPG. Also, check into the overall record time per MB of disk space. With any removable media, you can carry extras and keep recording as long as there is battery juice, whereas a hard drive's capacity is finite.
Whatever you decide, make sure it has an AV input before you buy if you you plan on attaching a helmet cam.