This is a great discussion, thanks to everyone for their thoughts.
I read the first couple of responses yesterday, and thought, "Okay, I'll get a PS4" but now reading everything else I'm all confused again.
The Wii U's appeal is that it has the Mario-based games, which seem ideal for a six-year-old, but it's being phased out for the NX, so it would be best to get that used from what I understand.
Both PS4 and XBOX One seem to have their advantages, so I would have to flip a coin and focus on the LEGO and sports games for whichever console I get.
I see that they come with a certain amount of storage. Would it be advantageous to go with 1TB over 500GB or is that something you'd have to be a heavy user to need the extra storage?