I really don't think you will get a good answer until someone receives a march build 6 series.
In my opinion, you should make the decision to either use the holiday incentives and order now, wait until the my2014 comes out, or move on from bmw all together.
My reasoning for this is, if you wait until someone receives a march build and it does have the new idrive, you car will at the very least be a late april build and wouldn't get in until may or june. At that point you are only a couple months from the my2014.
And if you order a March build before really knowing and it doesn't come with the new idrive you are going to be really disappointed that you waited and didn't take advantage of the incentives.
You should also probably go talk to the people in the m5 forum to see if they really think the new idrive is worth the wait. The hardware specs are a little better and you get 3d maps, but it really isn't anything astonishing.