The Gamma AR is the better one for use as a midlayer. The Gore Tex softshell Scorpion jacket you have doesn't breathe as easily as ProShell or Paclite, so I think you would be creating problems for yourself by layering with another breathable membrane. Unless you are taking lazy laps, you will probably overheat. The AR will layer better since it is a bit more flexible, it will breathe quite a bit better, and it will still provide you with a thermal layer. Go with the Hercules if your hill is a -30-on-a-sunny day icefield. The high-loft fleece Hercules, as a midlayer, will keep you warmer, breathe even better, and, thanks to that high-loft, provide a little extra padding if you catch an edge and bellyflop onto blue ice.

if you're deciding between these two, the Gamma AR will be much better as a layering piece. You should look at the Hercules, though, which is ideal as a layer, and a medium protection outer softshell.Edit (Jersey Hiker):Keep in mind that an underlayer with a smooth face fabric (hercules, Tau or Rho ar) will slide smoothly under the Scorpion. Regular fleece layers tend to bindPersonally, I would steer clear of any jacket with a wind membrane as an underlayer for a waterproof shell - it's unnecessary and can kill the breathability. In my opinion, the Hercules could be overkill since you already have some insulation on the Scorpion, but if you run cold then maybe that's ok with you. I would go with a more conventional fleece with an open face fabric.I agree 100% with the previous answer. I have the Scorpion and use the Rho with it or Ether Comp jersey.