I wanted to second what Max wrote. The D's rarely flutter. And they are way different kites than the C.5 version. Also, I agree with 3inletswindsports above on the advantage of one or no strut kites having an advantage in low winds....staying aloft. But i do not think the advantage of these kites is lost above 14knots. I perceive marked differences between the small strutless D Clouds (and single strut D_1 that I tried a number of sessions) and comparably sized other kites. Fasting turning, massive immediate depower, more apparent wind and better drift are still important issues at 18 knots to me. I recently switched back and forth between a 6.2 D Cloud and a 6-ish meter 3 strut kite...in 14 to 20 knots, surfboard and hydrofoil. There was a world of difference. I preferred the D (it also much more smoothed the punchy wind). Others might prefer the other kite, but the point is, they were way different.(To me, multiple struts had an apparent disadvantage even in the higher winds....for my hydrofoiling purposes)
Also, I would not assume that two strutless or two single strut kites have very similar performance characteristics. Just as three or five strut kites vary tremendously, surely strutless kites vary a lot, despite the similarities. I find the different Cloud models themselves vary tremendously and I even tend not to lump them together. Ideally, you could try out different single strut/strutless kites and see what suits you.