I think you should definitely read from Elantris, and I can't off the top of my head think of a better place than the beginning. When you are a huge author like Lois McMaster Bujold and most of the people at your reading got your newest book the first day it was out, then you tease them with the next book. But in your case, you would certainly hope that a large percentage of the people going to your reading have not read Elantris at all--this is an important time to market it to them.
It would make sense to read from Mistborn after January, if you're going to any cons between then and when it comes out, assuming it's still coming Mayish (though you're rather behind on production compared to Elantris, if that's the case, right?).
The new YA book is fresh in your mind and seems new and exciting to you, but by the time it comes out (if ever) anyone who hears you read from it now is going to have forgotten everything about it, whereas Elantris is fresh and exciting to anyone who hasn't read it 15 times over the past 6 years already like you.