I have been running an M3 for the past couple of years, and really like it. I am also waiting on a couple of T-1's to arrive, hopefully today.
Aimpoint lists the respective weights at 3.7 oz. for the micro, and 7.8 oz. for the M. I just weighed my M3 with ARMS 22M68 cantilever, and it is 406 grams [14.3 oz.]. I also weighed a LT-100 ACOG mount [this is the only LaRue mount that I currently have without an optic attached], and it is 96 grams [3.4 oz.]. Considering that an LT-660 is probably a little lighter than the LT-100, one could expect to shave approoximately 1/2 pound off their weapon system using the micro versus the M, including mounts. Some might not consider this particularly important for an AR platform, but one of my micro's is slated for use on a Buckmark pistol, something that I would never consider using an M for.
The reduced lens size should not be of negative consequence, since light gathering capability is not really a relevant function for this type of optic, and the EP's of both are way above what a human eye can make advantage of.
I am interested to see whether I like the 4 MOA dot better than the 2 MOA, but I don't think it will be of much consequence either way considering the applications.
An interesting point to consider is that Victor the NV guy, has historically favored the EOTech over the AP due to reduced view obstruction of the housing. It is my understanding that he now favors the micro over the EO for the same reason; that should say something, even if one is not employing NOD's.
Needless to say, I am eagerly awaiting the arrival of my micro's, and will be posting a comparative review in the not-too-distant future.
Hope this helps with your decision.