I think there was a thread on this not too long ago. The Bell Sweep R seemed to get the nod over the Giro Atmos. It truly is a personal fit thing. I was plenny happy with my old Specialized Piranha until I tried the Atmos. The Atmos is so comfortable that it just disappears on my head.
Or is that, my head disappears within it? I dunno. Prolly the latter. It's likely you can't go far wrong either way. Try 'em both.
MIke in AR