I voted for Top of mask. My crew recently got a GoPro, and we had a practice day to figure out how to use it before we go to a big game next week.
We primarily only used the Barrel mount that day b/c we didn't want to perma mount the attachment to our mask.
Two downsides to Barrel mount (that i have had expereince with).
First: Most players play w/ their barrel pointed down most of the time. I do not, so i took the burden of the cam. (which leads to point two)
Downside with the mask mount, if you run a fan...remeber to turn it off when you don't need it "hrrrmmmmmmmmmmmuuuummmmmmmmmmmmmmmmmmmmmmuuummmmm uummmmmm" is all you will hear the whole video...
Second, the circle clamps that came w/ the one we got absolutely kill all sightlines along your barrel. Find a way to get the little clamp screws out of your view and it might be okay.
Also, looking for some advice from you GoPro experts, what settings to use? We initially didn't change any of the base settings, we just hit "record". The results seemed a bit too wide-angle and not very easy to see whats going on down-range. After looking through menu, i noticed a "narrow" and "medium"...does that help it be a bit zoomier and less wide-angle?