I've heard both Magnepan and Final Sound, though both on seperate occasions, in different stores, and driven by different gear - so take my comments with a spoonful of salt. But I did get to hear the same song at least!
I heard the Final Sound Model 600i and the Magnepan MG 3.6. The Magnepan was driven by some very nice McIntosh gear, while the Final Sound was driven by a flagship Denon Receiver (can't remember the model) - I was baffled that they would hook up the very expensive Final Sound's to a home theater receiver, but hey - that's what they did.
Both stores had the speakers set up in untreated rooms very similar to a typical 20x25 living room. Songs were jazz by Norah Jones, classical by Holst, and opera by Carl Orff.
On to the impressions...
Imaging - I emember the Final Sounds having more of that laser-precise imaging when I was in the sweet spot, but I could definitely hear the sound lose some of that laser precision when I moved my head around (so don't lean on either elbow when listening)
The Magnepans imaged very well also (better than any of my home speakers), but were a tad less uber-precise - it was more a wall-of-sound with really good seperation as opposed to pinpoints of sounds coming from instruments floating in space.
Soundstage - The Magnepans definitely won out here - they threw a wider stage than the Final Sound's, especially on orchestral works like Holst. The Finals Sounds were good too, but not quite in the same league. I think the Magnepan's "wall of sound" ability also helped them win this one. In terms of soundstage depth, the Final Sounds precise imaging definitely helped you pick out what intruments were in front/in back as well as where they were from left to right. The Maggies did this well, but also seemed to impart a feeling of the scale of the room that the recording took place in (i.e. - On Holst you could really get the reverb/feel of the concert hall on the Maggies, whereas with the Finals' it was not as noticeable).
Bass Extension - Again the Magnepans won out here, but they are spec'd to go about 10 hz lower, and were being driven by a very hefty McIntosh amp, so bear that in mind. The Finals Sounds went low enough that I'd consider using them without a sub for most acoustic music.
Treble Extension - The Final Sound's seemed to have the edge here - upper frequencies were pinpoint in placement and were more noticeable on things like cymbals/triangles/drums/etc. Both Maggies and Finals did equally well on human voices.
Magnepan Sonic Signature - If you like your sound on the smooth side, the Magnepans will deliver. I prefer my music this way (but not my movies), so I would probably pick Magnepan as a speaker to live with (although the Finals would be fun for analytical listening sessions). The Maggies also sounded like they had a bit more low-end bass than the Finals, but that could have been the McIntosh amps. The Maggies also deliver the wall-of-sound experience, which I found created a sense of the scale of the recording on orchestral peices, while not sounding unrealistic on smaller instrument groupings either.
Final Sound Sonic Signature - If you prefer a more analytical, laser-precise kind of sound (though not fatiguing), then the Final Sounds are for you. They definitely had more precise imaging and could create the illusion that the artists were in the room with you a bit better then the Maggies (though the Maggies do this trick as well - it just wasn't as extreme). For me the Final Sounds were almost scary when I first heard them - I've heard good imaging before, but this was to the point where your ear is insisting there's someone 5 feet in front of you, and your eyes are telling you "no there's not". Creepy if you're not prepared for it.
Hope this helps!