It's really up to you. A lot of people told me to wait before buying the 12.9" one in mid December 2016, and even though I have till the 24th of this month to return it I still plan on keeping it. I think the only way I might return it is if an announcement is made before than. I seriously doubt this will happen.
By the way, I bought the 9.7" pro the day it was released. I still have it in the drawer and it hasn't been used since I got the big pro.
Sent from iPhone 7 plus with the 15" rMBP sitting next to me.