In my sytem's context which includes a fairly small room (12'x11') and a low powered amp. (First Watt F4) the main difference is in soundstage and instrument size which the Spatials do better. Also because they are OB depth perception is more believelable. The Quads offer smaller soundstage. Their soundstage is entirely behind them which is kind of cool, which I guess has to do with the room and my close proximity.
The Quad's high frequencies are more defined and refined in my view, female sibilance is controlled better and I perceive more air. Soundstage and instrument size is affected by the low watt amplifier which is suited better for the Spatials, and of course the small room size. I have tried a few higher powered amps lent to me with the Quads and these resolve the soundstage and instrument size issues. Microdetails in the Q's are more evident at lower volumes.
Bass response with the Q's is stronger, deeper and better defined. I've read some posts saying that the Quad 63 iterations do not do bass too well and I believe they do pretty well in that respect, at least in smaller rooms.
I do enjoy both speakers and are rotated regularly. And, the Spatials look much better in the room. So both speakers are very musical taking into consideration their different designs. So if anyone has the M25 tweeter option I would like to hear from him. Thanks.