I kitesurf and wakeboard. I find that both compliment each other, when there is no wind it is wakeboarding and the opposite if its windy.
It depends what sort of wind conditions are in your area. Kitesurfing you can generally get out and about in less wind (you obviously need a bigger kite to do so) and shallower water as there is nothing bit little fins sticking down.
I would say there is more variation in kitesurfing too (I might be slightly biased) but there is wave riding, trick riding and just plain old out on the water chilling as well as getting something for the land like buggies or landboarding.
As for cost it can be pretty expensive (but i would think windsurfing is too) as a minimum you would probably want 2 kites to cover most wind conditions.
Another thing is that you become slightly sad in that you are constantly monitoring the weather reports praying for decent wind in the right direction!

which like this year can become annoying!
doddi