I think if I had seen the Launch Trailer in front of AVATAR, I would have pissed my jumpsuit! But regarding which one made more impact, I guess it's just a matter of personal preference. I just absolutely loved the Launch Trailer because of it's emotional, cinematic value, and it's sweeping music that just took my breath away. When I saw the E3 trailer, I was blown away- but not in the same way. When I saw it, it was my (and everybody's) first real introduction to Mass Effect 2, and with the heavy drums and roaring guitar, it made me melt inside, wanting to see more from the game. The Launch Trailer, coming on the heels of the launch (obviously) made me absolutely die with anticipation- I had known who all of the characters were to some extent at that point, and It was exactly what I needed to push my over the edge into a blind frenzy on excitement (see the Mass Effect 2 thread around that time). And it had Martin Sheen's voice. Martin f**king Sheen. I was sold.
I've been a fan of Mass Effect since day one, and if Mass Effect 3 can have trailers or a trailer like that one- my head will explode SpaceMarines style.