We lived on Lamma for 5 years and moved as soon as we had kids!
The medical coverage is poor to non-existant and the playgroups etc for kids are very cliquey. All the bars and restaurants are smokey and there is nowhere to change a nappy if, like me, you are one of those people who don't think it is fair to change a nappy in the middle of a restaurant.
That said, everything else about Lamma is great regarding greenery, beaches, walks etc but you have all that and more in Sai Kung.
We live just before marina Cove so are still close to the dual lane road and thereby avoid weekend congestion and are closer to the MTR. I work in Sheung Wan and my door to door commute is less than an hour, something I can live with to be able to be a few minutes walk to to the beach, great swimming pools, playgrounds, soccertots, playgroups (organized or with groups of people you invariably meet within a day of getting to Sai Kung!).
I car pool or take the bus though my wife chooses to use the car as it is more practical than taking a taxi or bus everywhere.