Hi mc,
Hawaii' season starts in early May and finished at the start of September. The trades can be a few weeks either side of the start of May. Best time to go is June/July where you will definately kite 6 of 7 days in good winds.
Maui is strong and gusty wind expect to ride on nothing bigger than a 10 or 12 m depending upon your weight and you could be on anything down to a 6m. You can ride at Lower Kanaha which is the main sailing/kiting area but you must rig and launch downwind of lowers down at Action beach or Pro Beach. Pretty sure you can rig from Canoe beach downwind to Pro Beach. Downwind of Pro beach is Action Beach and downwind of that is Poo beach (near the sewerage plant).
The water is often choppy and can be crowded where the pro's ride with the exception of the miniscule amount of flat water behind the rocks you see in all the vids. Kihei on the other side can be really nice and gets a small wave, but also can get crowded on weekends.
Unless you are kiting at the top of your game and are extremely confident of your abilities, stay clear of Ho'okipa and Lanes,(Ho'okipa is the mecca for windsurfing and yuou wont be welcome there). Lanes is fun but is the most treacherous launch I have ever endured (and thats sayin' something) in fact that whole area of coast all the way from Mama's fish house around past Sprecklesville to Kanaha is a no go zone due to the airport which only leaves the Lanes launch. Sharp reef and offshore crazy gusty wind every where you go that part of the coast.
Oahu is less windy but much steadier. 15-18 knots everyday in Kailua Bay with occasional days into low 20's. A trip across the island to Mokulaiea will give you daily winds of 20-25 knots and as steady a breeze as you will find anywhere in the world, nice waves too. Very crowded on weekends but great midweek. You'll need a car.
I would suggest Oahu over Maui as there is heaps of other stuff to do as well as kite and it has a nite life where Maui has nothing. Crowd is friendlier on Oahu with less attitude compared to Maui.
You can also kite the big island and Molokai.
Hope this helps you, I had two great seasons in Oahu with trips to Maui and for me, Oahu rocked!.
Good winds,
