Yes, I have considered the 6D, but in reality its advantage over the 5DII does not justify the added cost, at least to me. While the focusing system on the 6D may be a "tad" faster or more accurate, it's still lacking compared to the 5DIII. Also, low-light shooting is something I don't do without artificial light. I do take photos of the Auroras all the time, but don't need more than 400-600 ISO (most times 400 is plenty with a slightly longer exposure).
Ah the joys of living in Alaska, never saw one single Aurora in my life