I think Kinect depends on what your kids like to do and perhaps age, my 360 is predominantly a Forza player but a friend gave me his Kinect and my 6 year old daughter loves to play dance games, kinectimals on that due being on TV etc. Only problem is explaining how to navigate menus.
I was probably just unlucky with most of the people I ended up online with the times I got online, most often a Friday evening or Saturday morning. Oh, one very positive feature that I missed off the PS4 is the ability to play a game whilst it installs! That was by far my favourite thing about it. On the subject of the Kinect, it definitely is a Marmite peripheral. The opinions are very divided on it. I mostly love being silly when playing on my own or with family and often get a lot of kicks out of the funny poses you have to do in some Kinect games. Me and my niece often end up on the floor laughing are heads off with the photos that it shows are you finish playing. That's always a good option. My preference over the external option is mostly because I like the idea (though not sure if it would actually work) of installing games on the external HDD and then when I visit a friend who has the same console, login my account to their machine so we can theoretically play both my pre-installed games via the external HDD and their local ones when we switches back to their account.
I can't imagine it is going to work that way for popping around your mates, as it doesn't with a lot of players that handle DRM, often as a security measure the HDD ID is linked to a machine and its license to combat piracy.
Yeah that's probably going to be the case, I'm just a very so slightly hopeful that it'll work as it did on a selection of my 360 games.
