Same situation here!
My 3.5 year old also got an offer from both schools. Personally I think you can't go wrong with either. But in the end, we went with CDNIS for the reasons everyone has already mentioned above:
Happy and nurturing learning environment
Full IB (so no need to worry about transitioning into MYP)
Better student mix
Abundance of ECA programs
Strong Chinese program - maybe not as strong as ISF but still pretty good for IS standards.