In reply to kmjohns1234:
I've used both, of the older models in each though. Sportiva have also just released a new OlyMons (with ski attachments in the toe area!).
OlyMons traditionally fit narrower than the Millet, and I found this to be true. I actually ditched my Millets because they were too sloppy - too wide and the insulation packed down even further, making them impossible to get tight. The OlyMons were more trim fitting and lighter, but I think the Millet had more insulation and were a bit warmer. I used both models multiple times in Antarctica, on Vinson and other high peaks, but only the OlyMons at high altitude (to 6900m on G1).