Even when the question is put that simple, I still have to answer in a different way
For me, and many I believe, it is not a question about one or another - as they are not only two extremely different "sports" I imagine most of us love, they are also mutual exlusive (almost), meaning not spot on in the same conditions.
When I can ride smaller than an 8 m2 on my waveboard, I need a 4 m2 and smaller on a surf foil, meaning, that is the only small range where one could do both, and none are in their sweetspot really
BUT, in reality, when wind is onshore waveboards sucks bigtime, and a foil with a 3 to 4 m2 kite a lot more fun.
When the wind is really okay pretty sideshore, and 7 m2 or smaller for waveboards, you REALLY enjoy this, as in most spots it does not happen every day...
Besides the fact, that most dont have such small kites, AND, they dont behave as well as bigger kites 5 m2 and up, IMO.
Also, a bit more onshore actually works better in waves with a hydrofoil is my experience, so again, mutally exclusive conditions.
If I had conditions suited for both (almost impossible), I would not choose one over another, but change for the variety.
Guess we all know that we are out on waveboards in okay but not perfect conditions, and thinking we would love to be on our surf foils instead.
AND, the opposite, having foiled a lot, and wishing we had better waves and out on our waveboards instead ha haa
If I could only choose one, it had to be foiling, but primarily because of its much wider range in many conditions, except severe seaweed where it is useless.
Otherwise, if both at their sweetspot, it does not matter which really