joomington2240041 wrote an answer about Arc'teryx Aphix Insulated Hoodie - Men's on September 11, 2012
The Atom LT has hand pockets too...
Other differences: The Aphix seems to have a longer cut or at least less of a drop-trail than the Atom, and it seems to be made from more a traditional "shiny" nylon fabric that are also potentially more wind-proof? Also it has 80gsm vs. 60gsm Coreloft, so it's warmer.
So in summary I would say: warmer, heavier, shinier, more windproof, less breathable?