Part of the answer to this lies in whether Disney is still running holiday entertainment and hours and is still fully decorated for Christmas and also how many schools are still on their winter breaks. I would think you can expect the parks to be very busy this year because many schools will still be out, and this year holiday time at Disneyland is scheduled through January 6.
I have been over New Years and I have been the days following NY. There is a definite difference in crowd levels after the holidays. With that said, we are going again this year during the NYE time frame. Just factor in crowd levels to your planning