Honestly, I don't see it as a competition. They each have their own pros and cons on any given day.
Personally, I've been picking up kiting to cover me on those sub 15kt days that I used to have formula gear for. I'm having a blast, and love not having to haul the massive kit around anymore. When the wind starts pushing 20kts that's when the windsurf gear comes out.
I've actually replaced 2 sets of windsurfing gear (formula & light wind slalom) with a 12m Lithium and V2 Sector and can't believe I waited so long to do so (although glad I did due to advances in safety tech).
So yea, I would be so quick to write one off in favour of the other. I plan to keep doing both for as long as the body will let me.