Macro is my very favorite type of photography, in fact the 100mm f/2.8L was one of my first purchases upon getting my first camera. I couldn't recommend the 100mm lens any more. The color, contrast, bokeh, and sharpness are all stunning right out of the box. I almost wish I didn't have the lens just so I could buy it again, that is how much I love it. Also, it might not matter in most instances, but since you are interested in macro it's worth mentioning that you'll only ever want to get closer. 1:2 won't be sufficient for long.
Never having tried the Zeiss, I can't really speak to how they compare, but I do know that the Zeiss 135 that I've rented, and the Zeiss CP.2 cine-lenses (18mm, 35mm, 50mm, 50mm Makro, and 85mm) that I use for work are all great, but take an additional level of patience and skill to use. That is not a bad thing by any means, just that when I'm going out to take pictures of everything from Landscapes, to Portraits, to Street Photography, to Macro work, I am more likely to grab my 100mm f/2.8 than any other lens.
I know a lot of other posters here use a tripod and focusing rails, focus manually, and don't rely on IS, but I like to go handheld. I chase insects mostly, and they don't really like sitting still waiting for me, so I need to move around a lot.
Most of the photos in my portfolio are taken with the 100, although the Macro stuff is taken with a mix of the 100L, the MP-E 65mm, and a reversed 50mm f/1.8.
Here are some examples...http://www.edwardboe.com/tabletop/
(all but one are taken with the 100L)http://www.edwardboe.com/portraits/
(all are taken with the 100L and the 135L)http://www.edwardboe.com/nature/
(roughly half are taken with the 100L)
Maybe rent both of them over a weekend and run them both through their paces. Either way, macro is super fun!