You kids are at the age where it is very hard to secure school places in international schools, even with company support. Is your company willing to pay a corporate debenture to jump the waiting lists? Even with this, you are going to have some difficulties and stresses if you are coming here in only one month and you still dont know which school your 6yo will be going to. If I were you, my questions would be all about schools and not mid levels vs north point.
In any case, to answer your question, basically mid levels is more 'cool' and you can impress your friends by saying you live there, whereas North Point is just a normal area with nothing particularly special about it. Apart from the coolness factor, ML is also more centrally located, more expat oriented, and more expensive. I would not necessarily say that ML is more convenient due to lack of MTR and so many hills make it a pain in the ass to actually live there (depending where exactly in ML you actually live). I often think of people that live in ML as having more money/ego's than common sense.