From what I've heard not only is kiting easier to learn but also less physically demanding. This is because once in the harness in kitesurfing you basically only need to trim your kite which requires very little effort. In windsurfing you need to can for gusts/lulls and trim the sail.
Also, I think in good conditions it is very unlikely that the kite should fall in the water, unlike a windsurf which requires consistent tacking and jibing skills to keep going. If you don't have these skills, uphauling/waterstarting can be tiring and time consuming. In this way kitesurfing might be less tiring.
I'm not sure about the effort/strength needed in legs for kiteboarding, though. Certainly in windsurfing when sailing powered up a great deal of effort is put into keeping the board under control using legs/knees.
I think that overall kiteboarding is probably less tiring and easier to pickup, though potentially more dangerous.
JP Super-X 106l, North F8 Ram: 5.8m,