I feel like the harlequin formations offer more to us than a cad. The Visarch and Yvraine are both a little underwhelming as HQs in my opinion (I like Yvraine well enough, but she only has access to two disciplines that aren't the most useful outside of an Ynnari detachment).
Harlequins are very expensive and pretty easy to move off objectives which makes them pretty poor choices for sitting on objectives.
I prefer running them in a masque/dual masque to get up in my opponent's face. Sometimes I'll bring a CWE CAD to provide more fire support as well as objective grabbing as they are better suited for it in my opinion.
For smaller games, the option to take a CAD is nice as our formations are so restrictive and can get pretty expensive.
That's my 0.02¢ anyways.