Newbee tells it like it is.
I've owned three pairs of Quads, five pairs of Maggies, and one pair of Martin Logans.
Here's the way I'd put it: The larger Maggies tend to have a relaxing and forgiving tonal balance, but are a little lacking in upper harmonic detail compared with the Quads. The Quads are (to my ears) a little bit forward in the lower treble region. Martin Logans don't have quite the detail of the Quads, nor the coherence of the Quads or Maggies, but they will play deeper in the bass and are in my opinion the most physically beautiful electrostats. Getting a good blend between point source woofer and line source panel isn't easy, and is somewhat distance-dependent, so I would say that either the Quads or the Maggies are going to sound more consistent from one room to another.
As Mrderrick notes, Maggies generally need to be cranked up a bit to really "come to life". If your listening style includes a lot of low-level, late night listening, then an electrostat might make more sense.
The good news is, I don't think there's a bad choice among these three brands. I'd call the Maggies the most forgiving with the widest sweet spot, the Quads the most detailed and revealing with a fairly small sweet spot, and the ML's in between with deeper bass but possible bass/panel integration problems.
Duke