Zeiss MTF seems a tad bit better than the Leica Summilux, although Summilux is sharper across the field, and both seem better than the new Nikon 58mm. In practise it may be difficult to tell them apart in pictures. Naturally for the Zeiss you're talking about a big fat heavy lens, so cannot compare with the Leica, who are tasked to design small compact lenses of high quality, and the limitations they entail.