-20C can do strange things your camera - you'll need to slowly heat/cool your gear to avoid internal condensation - weather sealing helps. Also, at -20C battery life will go way down, doing long time lapses can be problematic when the battery temp is so low. Keep the camera gear cold the entire time if you can (ie, in trunk/pickup truck bed, in cargo rack on top of car, etc.) - that way you can start taking photos as soon as you hop out of the car. I sometimes bring my camera with while backpacking/mountaineering - I'm a huge fan of weather-sealing, as it's just one less to worry about. That said, Ziess makes some really great glass, so the improvement in IQ might be worth it if you are willing to be extra careful while transitioning temperature zones - but I'd probably recommend getting something wider than 35mm.
The Canon 1.4/24LII is a pretty solid choice - I've rented one a few times for national park trips, overall it worked well, although I do agree with tron that focusing to infinity with the Canon in next to zero light can be tricky.
dpreview.com has a truly excellent article written by Ben Hattenbach on the subject of capturing aurora, if you haven't read it yet, I highly recommend it:http://www.dpreview.com/articles/8217618174/auroral-photography-a-guide-to-capturing-the-northern-lights