Well, the ZM 25 is known for being the most clinical lens in this line. Basically it has no weaknesses, the MTF are very flat, it has no CA, insignificant distortion, vignetting is quote good for this FL. OTOH, center sharpness is not sky rocketing, contrast is tame, 3D isn't as pronounced as with other Zeiss lenses. This, I know from experience since I own the lens.
How's the 21 in this respect? The MTF look more wavy with higher center sharpness, and I read somewhere it has more field curvature, causing weaker corners, and perhaps more 3D. What's the opinion of the owners? I would be very interested to hear it.