My guild has been raiding EC8M HM for a while and we often run with two Snipers, me as MM and Cildrnic as Lethality. We use the builds posted earlier in this thread.
You might find these logs interesting:
http://dpsmeter.com/lasted0loft
http://dpsmeter.com/bob0boil
Look at the kill fights for each boss since those give the clearest indication of our performance. Wipes doesn't give a clear image since one of us died before the other and other random stuff. Also look at which targets we did the most damage to, because these fights have many phases.
Lethality and MM seems to be very close to each other, with MM being dependent on armor debuffs to match Lethality. Lethality seems to do good in movement phases and doesn't care much about armor debuffs. Ultimately your spec is more of a preference thing. I have raided as Lethality a couple times, but it's not my thing and I'm geared for MM, which means that I don't need as much crit rate as Lethality, so I stack Power. We have more logs, overall we're neck in neck depending on the boss mechanics.
I feel that MM is a bit more work to play optimally than Lethality, which is reflected by my APM.
My stats buffed and stimmed:
Cunning: 2119
Weapon dmg: 1056 - 1148
Bonus dmg: ~643 (Power 704)
Crit rate: ~34.7%
Surge: ~75.6%
Cildrnic has stacked Crit rate to around 40% and Surge is around 72% last time I inspected him, he has sacrificed a bit of Power to achieve that crit rate.
Hope this helps!