In reply to shibz:
I have both. I picked up the atom for full price and got the prism for £40.
I'll be getting another atom lt when the time comes. Not to say the prism is bad but the atom is lighter, warmer and more breathable.
The prism has the better hood though and a two way zip.
I'm 5'10" and 12 and a half stone. The atom fits me better, the prism comes up too short in the arms and waist. Both are medium.
The better sizing could account for the better warmth so you may need to try them both on.
Definitely true about the prism being less precious but the atom is surprisingly tough too.