I have an M512. It's heavy as all getout. I don't use it anymore, as I bought a Soundcraft Ghost. The 512's low end blows the Ghost's out of the water, but the top end really sucks. You have to crank the high EQ knob to get any sparkle. Sometimes I miss that fat low end, but all in all can't see the 512 as a viable option.
I have no experience with the Yamaha.