Any reason to buy this instead of the older version, which uses Polartech Powershield?

Arc'Teryx experts on their site actually answered this. They said the new Fortius Fabric is much more resistant to fade as well as has improved breathing properties. They also confirmed because it was designed to increase breathing, it is not as wind resistant and possibly not as warm as the prior model. The Fortius 3.0 fabric in the Gamma AR is simply thicker and as such, is closer to the the prior Gamma MX

Dean, are your weight and materials specs for Fortius 1, 2 or 3?

Hey there, no not really, unless you are partial to Polartec brand... here's the scoop on the all new Arc'teryx created Fortius material: Why the new fabric? It's made in a small Japanese mill and means Arc'teryx Jackets has more control over its specifications than they would dealing with a larger brand like Polartec. Ultimately that means better performance, they say. It comes in three versions. Fortius 1.0 is a stretch woven fabric for high energy stuff, then there's Fortius 2.0, used in the classic Gamma MX and AR lines, which has a polyester brushed liner laminated to a stretch woven nylon face fabric. The adhesive used in the lamination process adds wind protection but is still air permeable and very breathable. Finally, theres's Fortius 3.0 with a warmer liner used for the Gamma SV and aimed at real cold condition use. Here's a breakdown of the specifics of the two materials: Fortius Fabric is a stretch woven plain weave fabric made of continuous textured nylon (84%) and spandex (15%) filaments. It is extremely lightweight, weighing only 160 grams/m2 (4.7 ounces/yd2), breathable and quick drying. Polartec® Power Shield®: A tightly woven nylon exterior face laminated to an inner insulation layer. Between is a breathable, discontinuous membrane that transports moisture away from the body. It is treated with a DWR (durable water repellent) to shed rain and snow. Since its directly designed by Arc'teryx personally it makes the new material sound very promising...