I have just had a look at the MTF of the Nokton (
http://www.imx.nl/photosite/technical/highspeedlenses/t003.html) and Sonnar (http://www.zeiss.com/C12567A8003B8B6F/EmbedTitelIntern/CSonnar_1.5_50_ZM.EN/$File/CSonnar_1.5_50_ZM.EN.pdf) and to me they are quite similar wide open...
One thing that attracts me to the Sonnar is its beautiful (to my eyes, at least) color and texture rendition...It is sharp without being aggresively so ("clinical"?). It almost looks paint-like.