Hi Matthew,
10.3 will ship with Python 2.7, likely 2.7.7, but that could change depending on where Python is leading up to the 10.3 release date. We didn't want to make a break within the 10.x release line. Pro, on the other hand, will include Python 3.4.
For the most part, arcpy is largely moving forward to Pro intact. From 10.3 to Pro, the biggest difference will be to arcpy.mapping because of significant architectural difference between the two products. There will be a few other changes as well. There will be a new tool in Pro and 10.3 that will help people review their code and tools for these differences.
-Dave