In reply to ElBarto: The Gamma LT jacket offers more durability and weather protection than the Squamish. It's also a closer fitting garment and is warmer in use. The fabric is a lightweight stretch woven textile which is called Fortuis 1. Arcteryx use several weights of Fortuis in there Softshel line up. A great piece for cool weather crag use, layering under a shel in crap conditions etc. I wouldn't want to run in it though. This is where the Squamish comes in. A much lighter textile is used here. It's not as warm and will breath better than the Gamma LT whilst you work up a sweat fast. The Squamish is more wind resistant than the Gamma LT but not as warm or durable.
Both great items with there own uses. You could use the Squamish over the Gamma LT too. It's a slightly less fitted garment than the Gamma LT.
I've been using a Gamma Hybrid Hoody which is fantastic and almost blends the best attributes of the two garments here. Hope this helps?