I would absolutely go for the 789, for these reasons, many of which were mentioned upthread already:

• Two aisles allow for more movement
• The larger plane feels less crowded overall
• Better in-cabin atmosphere due to higher pressurization of the composite 789 vs the aluminum 752. It's also more quiet
• Overall, the 789 just feels nicer, on account of it being newer
• Entertain your child by futzing around with the smartglass windows on the 789
• Larger galley space in the back