I am looking for a mid layer for colder weather that can also serve as an outer shell for warmer weather for the Appalachian Trail. I am torn between the Hyllus, Tau, and Gamma MX. Any suggestions or advice?

Angel, thank you for pointing out my error. I did mean the Atom LT not the Alpha LT. For some reason Alpha got stuck in my mind. Anyway, I have corrected my original post.

ng2010, do you mean Atom LT instead of Alpha LT?

I own the Gamma MX (hoody) and the Hyllus and have worn the Tau. However, to address your need, I would recommend the Atom LT. The Atom LT features a water and highly wind resistant (although not windproof) nylon face fabric over a light Coreloft layer covering all parts of the jacket except the sides and armpit area where it has Polartec Hardface fleece for increased breathability. The jacket also has an athletic fit facilitating its use a mid-layer. It is about as warm as a mid-weight fleece (e.g. an Apache), but obviously does not suffer the wind permeability that a fleece would, so it can function quite well as an outer layer if need be. See below for a comparison of the three jackets that you mentioned: The Hyllus is substantially warmer than the Tau and Gamma MX. For example, in calm to light wind, the Hyllus along with a long sleeve polo shirt keeps me comfortable down to about the mid to high 30s when engaged in little to no activity. Obviously, higher output activities would require substantially colder temps to prevent overheating. In calm to light wind, the Tau is slightly warmer than the Gamma MX, being equivalent to a Gamma AR. With little to no activity, the Tau will keep me comfortable down to about 50 degrees with only a long sleeve polo underneath. Under the same conditions, the Gamma MX will have a minimum comfort temperature of a few degrees higher. The Atom LT will be slightly warmer that the Tau. I do not believe that the Hyllus or Tau would be a good choice for a shell while hiking due to the following considerations: In a windier environment, the Tau and Hyllus will likely be unsatisfactory as outer layers because they are too wind permeable. In contrast, the Gamma MX, while not wind proof, is highly wind resistant. Additionally, although the Tau has a DWR treatment that will resist light rain and mist, it is not as effective (based upon my under-the-faucet tests) as that of the Gamma MX, which is quite water resistant. I cannot speak to the water resistance of the Hyllus. Finally, the Tau is just a fleece, albeit with a smooth face, so its durability will be substantially less than that of the Gamma MX. I would expect that Hyllus, would be less durable than the Gamma MX too, but I never subject my Hyllus to rough treatment, so I am not certain. Based on the above factors, you should consider the Atom LT as a mid-layer and a Gamma MX hoody as a shell. I think that they will work well together.