I've sailed the Serenity in a very wide range of conditions.
20-25 knots with a 4.2 m2 Sailworks Hucker (the Serenity gets "scary fast in flat water with a 32 cm Tangent Reaper weed fin in these conditions) on very flat water and it was super "exciting".
I've sailed the Serenity in 12-16 knots with 5.6/6.6 Huckers and 7.2 m2 SW NX slm, as well as with the 7.5 m2 Severne Glide.
That's the beauty of the Serenity, you will never "slog" as long as there's at least some wind, but you can sail in all conditions up to where you want to start sailing your Flow and be "challenged" throughout the entire range.
Beyond 15 knots, you Flow may be more fun, as you can only go so fast on the Serenity, but even when it's maxed out, it's still challenging.
As I said before, I've sailed the Kona, and if you wanted to go racing, the Kona would be a better choice as there's now a Kona class at most races in the USA.
If you just want to free sail, the Serenity is probably better as it will sail circles around the Kona in < 6-7 knots of wind.
The Kona will be in slog mode in < 6 knots, but the Serenity will go nicely, and will not require a huge sail to do it.
Hope this helps,