A1 : No Idea
A2 : As below, in my opinion.
Helmet Cameras
Benefit of Helmet Camera is it see's your Point of View, as your head is usually looking at the subject in the video. Exception being rear ended as you can't see what's going on behind you. Helmet camera's usually have a wired microphone so if you want to 'VLog' or record your chat, these are better.
Downside of Helmet Camera is internal battery life. Its a pain in the ass to swap batteries and worse remembering to charge them in the first place.
Bike Fixed Cameras
Benefit of Bike Fixed Camera's is you can have more of them to record FRONT and REAR footage. Also you can hard wire them onto your bike, so they always have power no matter how long you are out, AND you don't have to remember to charge batteries if you hard wire it. You can also have voice chat if you pair a bluetooth headset.
See my setup thread here for the Garmin Virb XE and Virb X if you want one of the best setups.
http://www.ukgser.com/forums/showthread.php/451559-Navigator-VI-and-Garmin-Virb-XE-Camera
Downsides to Bike mounted camera's is security. You need to remember to take them off or lose them.
Getting carried away with the best... DOWNSIDES TO Super HD 4K
It's easy to get carried away with wanting the VERY best video quality and 4K resolution.
But in my experience the file sizes are so massive it'll impact your usability of the footage. Laptop ability to render these massive files, storage costs to keep and maintain Terabytes of files, Download and Upload times to get your footage off your memory cards onto laptop and then uploaded onto YouTube.
I decided that 1080p HD was enough... and that has it's time, storage and upload issues as it is.
The biggest pain in the neck with any motorcycle camera's is the time it takes to download, render and upload any footage you want to look at. (next to remembering to charge batteries the day before and reloading fresh batteries regularly while out riding). It was the battery issue that stopped me using my Helmet Camera. It's just to much hassle charging multiple batteries every time you want to go riding, so I switched to hardwired bike mounted camera's