Well, I have heard them both, but not in the same system. I have heard the M2.2s several times, but the M1.2s only once, although that was an extended audition of a couple of hours. (So take what I say with however many grains of salt as you like!)
Basically, I agree with Maggieman.
The M2.2s have all the power you would (probably) ever need, as the 220 wpc will drive just about any speaker. and it has very, very good bass extension, which is tight and deep. (My friend has Rockport Antares speakers which go down to 20 hz, and the bass response with the M2.2s is magnificent!)
The M1.2s have a very, very similar sound to the M2.2s, but with just a touch more smoothness to the mid-range.
If you have efficient speakers (93+ db), and especially speakers whose impedance doesn't dip below 8 ohms, than I think the M1.2s will work great. If you use more typical speakers (i.e. 88 efficiency with an impedance of 4 ohms or so), I think the M2.2s will work better for you.
One thing though, that I recommend for either amp is to replace the stock 6922 sovteks with some nice NOS tubes. If you are stretching your budget (like I did when I bought my 200 wpc M2.1s, (the M2.2s predecessor)), than something like a Amperex Orange Globe 6DJ8 for less than a hundred dollars a pair will improve the sound quite a bit. For the ultimate improvement, I suggest something like a pair of Amperex 6922 pinched waist tubes. These cost a few hundred dollars a pair, and are slightly better than the Orange Globes. (I did eventually upgrade to the later tube after a year or so, figuring if I am spending several thousand dollars on amps, I might as well get every last ounce of purity out out of them.)
FYI, some people report that the sound of the Lamms are slightly dark. I think this might be true when using the stock Sovtek tubes, but not really true when using really good NOS tubes. (I compared the M2.2s, with said pinched waist tubes, against the DarTZeel amp, and there was very little difference between the treble reponse of the two units. I think the DarTZeel might, (maybe), have been a little more extended, but I preferred the Lamm, as it had a better cymbal decay, IMHO anyway!)
FWIW: If in doubt, I would go with more power vs. only very slightly better sonics.
Good Luck in your search!