Hi Max, what experience do you have with your Ether Thermo? What temps have you used it at, and what was your experience at these temps? This will provide a good reference for your upcoming trip.
Personally I would take both. A CCF pad is invaluable in cold weather and especially on snow. You can use it as a sit pad during breaks and in camp, and also if you're standing around, you will be much warmer standing on the pad.
That being said, I just can't sleep comfortably on a CCF. I use a Thermarest 3/4 over a CCF; this is a pretty popular winter setup. I've also used a Downmat 9, which is an inflatable down-filled pad; at 0F I was completely comfortable. Of course the R-value of this pad 8.0!, and it weighs over 2 pounds. Sleeping comfort, whether from a warmth or cushioning standpoint is a very personal thing. Make sure you choose a system that will ensure not only warmth, but allow a comfortable sleep without waking up sore from too little/hard padding beneath you. You're gonna need your rest for hiking!