I would add that the areas of Clearwater Bay and Sai Kung are where the NT expats are concentrated. Sai Kung is lovely, and la little town in its own right. Clearwater Bay is also very pleasant, but is more like a residential suburb that provides a quick train commute (MTR) into Central (But you have to drive to the station- 5-15mins depending whereabouts you live- CWB is one long road). Both have pretty good beaches. Have you called CWB ESF school?
The other thing is that HK Academy (which is a lovely school- currently on a temp site in Kennedy Town on the island) are building a new campus up at Sai Kung, opening in 2013. I went on their open day- the ethos of the school is wonderful, and they are going to have an amazing set up in Sai Kung.
There are also quite a lot of expats around Shatin. I don't really know that area though (only go there for the horse racing).
I agree with what Tyel says about parts of the NT being quite a way from the island, BUT it is a LOT cheaper and it depends on how much you're going over there. If you start off living in the NT, it's likely that you'll make friends out there and not socialise so much on the island. That said, I have friends who live in CWB who come to the island a lot, and say that they just factor it in to their night - it's easy to get a cab to drive you back. If you're driving, on a clear run, CWB is only 25 mins from Central.
You definitely need at least one car if you're living out there.
On the club scene that Tyel mentioned, unless you're getting a membership as part of your package or one of you is a competitive sportsman at a reasonably high level (so would qualify for a sporting membership), take that out of the equation because the waiting lists are insanely long now and individual debentures ridiculously expensive. I know no-one who is self funding a club membership.
The main benefit of NT is the cost. HK$60k per month will get you a pretty grotty 3 bed with no facilities on the South Side of HK island, but a house in CWB/ SK.