This is what Zeiss says about the ZM 50/2.0:
"Undoubtedly, the new high performance standard in M-system lenses. It is based on the famous Carl Zeiss Planar® lens design concept, a symmetrical combination of 6 optical elements in 4 groups. So count on exceptionally high resolving power, evenly distributed performance over the entire frame, and superb control over flare and ghosting."
This is what Zeiss says about the new ZM 50/1.5:
"It is fast, very resistant to flaring and unusually compact – therefore the “C” in the lens name. With its well controlled distortion, the lens produces a very accurate representation of objects."
If I were to believe advertising copy I would have to say that Zeiss thinks the ZM 50/2.0 is the better lens.
Ted