Hi Felix007, thanks for your question.
Unfortunately this is not a one word answer and depends heavily on what you want the jacket for. While both jackets use synthetic insulation to keep you warm they in fact are made for very different purposes.
The Arc'teryx Atom LT jacket uses Coreloft and Polartec Power Stretch fabric on the sides to provide some breathability and this is really the main difference between this jacket and The North Face Thermoball. The Arc'teryx Atom LT jacket is an 'active insulation' piece, meaning it is designed to breath while you are moving. It isn't totally windproof and while it keeps you warm it's designed to breath while hiking, climbing and ski touring. It's an excellent mid layer as well since it won't make you feel clammy when you get warm.
The North Face Thermoball on the other hand is more akin to a synthetic down jacket. It's designed to keep you warm when you are not moving, lunch stop while ski touring or belaying on a cold day. Both are very good at providing warmth, the Thermoball will buffer the wind a little better but neither are really waterproof. If you are looking for a mid layer that doubles as an active outer layer I would go with Arc'teryx Atom LT jacket, if you want pure warmth when standing around and more of an outer layer then the Thermoball my be the one.
If you like the sounds of the Arc'teryx Atom LT jacket then you should also consider the Patagonia Nano Air, the Marmot Isotherm, the Outdoor Research Super Layer and my personal favourite, the Westcomb Tango Hoody. There are also some other alternatives to the Thermoball on our insulation review page here.
The ThermoBall Hoodie is a new synthetic down jacket from The North Face. Introduced this Fall, the The North Face boldly claims to have improved upon the incredible qualities of down insulation with its proprietary ThermoBall insulation.
