I seriously comsidered the E as well. What does your experience withthe M9 tell you about when moire would be a problem? Do you think it just ceases to be an issue when the sensor approaches the resolution limit of the glass?
I look at the incredible detail of the MF backs and wonder if, at that resolution, the moire issues worth worrying about. Certainly fabrics would be one of the most problematic situations for moire, but fashion is where many of the MF backs work.