At the moment there is a very small difference between the two as well as very little reason to buy it, specially considering we have to rent it instead of the one time payment. I want to know what you guys think they should add while I will list my suggestions as well. Of course my suggestions are a little too much, but I would honestly be "renting" C3 when it comes out if it had any of these improvements over C2.
My Suggestions:
More Export/Import support (this would be nice for phone games)
Construct 3 for Android or other phones (for work while away from the computer)
Built-in Shading (technically there is shading is in C2 but it is garbage)
Concave Collision Polygons
Better Physics Engine (like co-operation between two pinned physics objects without bugs)