I owned Zi, M7 and M6. They are are all wonderful cameras.
I found AE faster and useful, particularly when you use AE lock (faster yet as reliable than manual metering). I use the M6 now, simply because it is the most robust, easiest to fix, and because it is not my primary camera anymore (I prefer the Fuji GF670 to all of the above), so I downgraded from the M7.
For me ZI and M6 would be too redundant. The ZI is more modern, faster in use, but a little less robust and louder (not that an M6 is silent).
Why not consider a Contax G2? Fast, very robust and stellar optics. I miss mine, and may go back to it.