I considered becoming a Quad dealer, but the new models to my ears don't live up to the standard set by the older ones (both of which I have owned). Of the two new models, I prefer the 988.
I haven't heard the Maggie 20.1's, but extrapolating from having heard the 20's and owned the 3.6's, I'd say you can expect better depth and power from the Maggies, and a more natural tonal balance. Detail and nuance will be better on the Quads, as will low-level listening. The Quads also give you a wider range of amplifier options, as they aren't as demanding of power.
The speakers I own and sell are Sound Labs, and I think they do some things better than either the Quads or Maggies, but they're even more demanding of amplification.
Getting back to your original question, if I had to choose between the Maggie 20.1's and the Quad 989's, I'd probably take the Maggies. And I'd take either one over just about any box speaker in their respective price ranges.
Best of luck in your quest!