In reply to Jackwd:
The Atom LT is not really suitable as a belay jacket at all. It is designed as a lightweight, insulated mid-layer for use whilst climbing and is therefore cut to fit like one. I.e. it has a trim, long cut and excellent articulation.
It just about cuts it as an outer layer for climbing on cold frigid days, but due to the powershield underarms, it doesn't really do well in wind.
I believe Dane1 has a review on his cold thistle blog, highlighting these facts, but if you're after a dedicated synthetic belay jacket you need to look at the Atom or Kappa SV lines, which are far more suited as outer layers.
Needless to say if you use it right the Atom LT is an incredible jacket and has completely replaced the humble fleece for me. Now they've bought out the pullover version, I'm incredibly tempted!
As for the RAB Generator, that too appears to be more suited for layering rather than as a dedicated outer layer. Unless you mean the Generator Alpine jacket which would be much more suitable and is completely different from the Atom LT.
Wow, that was long winded.