Well, I have yet to spend time with the Quad 988, but I used to own its ancestor, the 63. And I've owned Maggie 3.6's.
Both are superb loudspeakers. They have different strengths and weaknesses. And we have different hearing - for example, a minor distortion that I am relatively insensitive to might drive you up the wall, and vice versa.
So I absolutely cannot say which speaker is best.
But I seem to be unduly sensitive to the very slight upper midrange/lower treble emphasis of the Quad 63's (and I do not know if the 988's have this characteristic). The Maggies to my ears have a much more forgiving voicing, even though they do not have the resolution and low-volume articulation of the Quads. To me, the Maggies are more relaxing to listen to long-term, and with a mediocre recording they do less to remind me of the recording's inadequacies. Very little of my favorite music is "audiophile approved".
To me, the listening experience is about closing my eyes and taking a journey into a different level of consciousness. It's like a very deep meditation, but with a lot less discipline required. Therefore to me, the speaker that does the fewest things to "pop my bubble" is the one I'd most prefer to live with long-term. To me, the Maggie 3.6 comes closer to this ideal than the Quad 63.
But I am NOT saying the Maggie 3.6 is better than the Quad 63 or the 988! I'm only saying that, given my quirks of hearing and listening style, the Maggie is a bit better fit.
Hasse, one of the things that bugged me about the Quads was the limited listening height - sometimes I'd get up to dance around, and the illusion would of course collapse. That's something I like about the big Sound Labs, which by the way are voiced more like the Maggies than like the Quads.
Kinsekd, you are choosing between two of the finest speakers out there, regardless of price. Neither is a wrong choice. One might be a little better for you, the other a little better for someone else. If at all possible listen to both. If you'd like a more detailed description of my impressions of the 3.6 and 63, e-mail me, though recognize that my impressions of the 63 don't necessarily translate over to the 988.
Best of luck to you!