Hi Damien,
Having listened extensively to both models in double-blind tests--but never side-by-side--I'd give a slight edge to the M22s over the M3s in soundstage presentation. The M22s are a little more "linear" than the M3s, a flatter frequency response overall, and they have narrower cabinets, which tend to aid in producing slightly wider horizontal dispersion.
Keep in mind that "soundstage" is also a function of the engineering and miking of the original recording, especially with classical and acoustical recordings, so a speaker with good horizontal dispersion and smooth off-axis curves that mimic the shape of the on-axis response will enhance the illusion of a broad spacious soundstage.
Regards,
Alan
