Well, I'm slightly biased towards windsurfing. I have kitesurfed a couple of times when the sport was just starting to take off.

If you have done neither sports before, the learning curve will be steeper with kitesurfing, You will get to a reasonable level quicker with kitesurfing, but the problem with this is that it doesn't give you a chance to build up experience and a sense about the wind and water. The injury risk is probably a touch higher with kite surfing for this reason. (Sorry no figures to support this just experience.)

Windsurfing, is a touch more challenging, but you get to go on the water immediately. It is getting easier to learn the whole time with improvements in equipment. (Certainly alot easier then when I learned 20 years ago!). If you are a beginner I would advise against buying gear and this solves the problem of having to carry it around. Places like Silvaplana have a rental center and school which as far as I know is reasonably priced. Once you can waterstart then it is time to think about buying some equipment based on where you are going to sail and what type of sailing you hope to do.

Personally the two boards I use in Switzeland are 110 and 82 litres respectively (Have a 63 too, but only get to use the during some storms here) . I haven't had to cart them around on the roof of a car yet as I had a van up until a week ago, but I don't think we will have any problem getting boards from Zurich to Silvaplana on the roof of the car when we do head up there during the summer.

You should be warned though, windsurfing in Switzerland is hardly an adrenaline filled extreme sport. It is more relaxed cruising around in summer breezes, or fighting the ridiculously gusty storms that they occasionally get. I spend more time dreaming about windsurfing then actually doing it here.