IMO, the Quark is worth it. It is smoother at low speeds, has an LVC, manufactured in the US (faster warranty that going to Germany I'd imagine) more programmable, and has an easily mountable case to protect its fragile internals. The only downsides is that it doesn't seem to like 6s lipo and it heats up without some form of secondary heatsink.
But if you already have the 9920 (and happy with it), I don't know if I'd get the Quark, unless of course $$$ is no object. :)
I have one on my Revo and it works great:
Pic1 Pic2 (I really should take better pics now I have a non-broken camers).