I’ve played with all of these (and helmet mount, all have pros and cons).
Helmet mount gives the smoothest video (your head is kinda like a steadycam), terrain is most flattened out. Looks kinda goofy with the gopro on your head though.
Chest mount is good middle ground, as long as you have it positioned correctly and have removed vibration from the clamp and have the straps as tight as possible – it’s reasonably smooth but terrain still looks flattened out. It’s uncomfy and sweaty though.
Bar mount is the least smooth but has a good feel for the terrain features, it also looks pretty fast due to being closer to the ground. It doesn’t look goofy and is not uncomfortable.
I’d recommend getting all three (dx.com is your friend – the k-edge clone is good value for £6 or so).