I just purchased the 6D on yesterday, so I will attempt to be non-bias in this response. I upgraded from my 10D, which I've shot with for 10 years. As you can imagine, I feel like I've been awakened from a long sleep! The first thing I tested was the ISO range. I took a few photos in my bedroom of my wife and kids as they were talking. The only light that was in the room was the night stand lamp (25 watts) and the ambient light from across the room from the television. Using the 50mm 1.8 at at 200 ish and ISO 5000, there was zero grain and the DOF was perfect. I allowed the camera to select the focal points, and I was fairly impressed with how fast the evaluative process worked.
Coming from a smaller a non-Full Frame system to the 6D was a frame opening experience! Being able to see what you've been missing, well, there are no words to describe that.
Like you, I shoot a lot of night club events, fashion shows and portraits. Tonight will be my first official shoot with the 6D. I am looking forward to not shooting with my speedlite all night! Larger pixels really do make better images. And the full frame 6D gives me the best of both worlds over my 10D. Again, I am comparing a camera from 2003 to 2013, which is hardly fair, but it is the only thing I have.
As for the weight - it is lighter than the 10D, and the 5 Mark III. Weight does matter when you are shooting for a long period of time. My arm and wrist tend to get tired after a long fashion show, so, those lost ounces will make a difference.
Overall, no, it's not a MK3, but it's not supposed to be. I don't need to shoot any faster than the 4.5 fps it offers since most of my subjects are stationary, or at least walking, not running, towards me. The Digic 5+ processor saturates my low-light photos far beyond what I could ask for in a $2000 camera. I am looking forward to many good years of use. And the wi-fi app is just icing on the cake. It works perfectly. Without reading any instructions, I was able to set it up in two minutes. The images that you download to your smart device are just a lovely as what is represented in live view on the camera. Needless to say, Canon has outdone themselves without destroying the MK3 market.
Sorry... I tried to be non-bias... it didn't work so well.
