Hi Mikito,
Water relaunchable foils provide more traction per square meter than bladder kites because they are double skinned. Their areofoil profile enables them to fly closer to the edge of the window, meaning that it's easier to stay upwind. They are also easier to setup and packup because there is no inflation required. However, the downside is that you have to deal with bridle lines. Although they rarely get tangled, this often scares people off. Alos, despite more engineering and sewing time, foils tend to be less expensive than inflatables - which is appealing to those starting out.
JoJo has a great line of water relaunchable foils: The Rage and Wet kites. They also have a new double skinned inflatable called the Acro. If you're interested check out our site
www.adventureplanetsports.com. We also have bladder kites by RRD, and Cabrinha.
Cheers, Dan