I have used both and here is my .02...
Leica glass in general has a unique look. It is actually of lower contrast than Zeiss yet very sharp. The lack of contrast can initially make it appear as less sharp than a higher contrast image, but this is not the case on close ispection. Zeiss OTOH has the clinical edge, which IMO comes from the contrast. Both are pretty neutral in color, but I would say the Zeiss leans to slightly cool while the Leica leans to slightly warm.
In actual use at f8, you will see little difference between these lenses resolution-wise. Wide open it will be a different story. The Leica is reknown for creamy smooth Bokeh, and though the 28 is not their best in this regard it does (again IMO only) have slightly smoother Bokeh than the Zeiss.
Finally, neither of these lenses is an overly "stunning" performer, but both are very good optics and certainly better than Canon offerings in 28mm.