I'm looking to pick up one of these helmets once they come on sale. However, I have a question regarding sizing. My head is measured as 54cm, which comes up on the very end of the Small size, according to Giro.

I picked up a Giro Prolight a few days ago, and while it was a nice helmet, the styling was a bit old for my taste. My problem is that since I sometimes ride with a cycling cap, should I go up to the Medium, or just stick with a Small? I can fit the cycling cap with the Prolight, but it is really snug and it also raises the helmet up a bit. So what do you think? I'm wondering if I should compare fit of the Prolight to the Atmos. I've checked a few LBS around my area, and not many seem to have the Atmos either.