I went to St. Lucia for a week 4 yrs ago (first week of January). I stayed at the Reef Beach Huts and kited at that location.
www.kitesurfingstlucia.com We kited 6 of 7 days (12 - 16 m 72 kg). Small chop.
The south area has two kiting areas separated by the flight path for the airport which is right there (but only a few flights a day). The other spot in the south is a few meters away from the Cocconut bay resort.
www.cbayresort.com
I did not kite in the north but I heard the wind is slightly less reliable and usually not as strong.
www.aquaholicsstlucia.com
If you decide to go north I believe the bus ride from the south airport is at least 2 hrs. There is also an airport in the north.
The acomodation at the Reef was simple but clean. (bug nets were provided but I bought my own).
The only downside of sleeping at the Reef was an outdoor DJ party close by almost every night. It didn't bother me as I was usually so tired I slept right through it but it bothered my kite buddie.
Food was excellent (reasonably priced with "package deal") at the Reef and many Yachties would stop by there for meals. My friend went the following week to the same place and kited on 9 - 12 m 7 out of 7 days. The Reef at the time also had small cottages for rent near by.
Summary: You should get wind with small chop. 9 - 16 m kites. If you are on a budget then stay at the Reef. If you want All inclusive Cocconut Bay. If you want more action then the North, but less reliable wind. There is excellent snorkeling and hiking on the island.