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The zeiss will only take you down to 1:2 and you will need tubes, filter, etc... to get to 1:1. That said the quality is excellent. The makro-planar T 100mm f/2.8 c/y does go down to 1:1, and is slightly cheaper. The bokeh of the Zeiss in my opinion is a bit nicer, and people generally laud the microcontrast. In terms of sharpness they are all going to be pretty similar, it is mostly the build construction, handling, bokeh and perhaps the most notable difference is the the number of revolutions required to go from infinity to 1:1. The zeiss has over 1 turn which therefore enables you to focus more precisely, however it will slow you down. The zeiss also is not automatic, and does not have IS. Instead it only has focus confirmation.
Overall the Zeiss is better quality but at ~$1500 and that's second hand it is pretty expensive. The only upgrade between the canon L and non-L is essentially the build quality. The IS doesn't play a significant role in the macro and won't find much use unless you're doubling it as a portraiture lens. IQ-wise you can save yourself lots of money by sticking with the non-L.
There are lots of other alternatives besides these lenses as well. You can reverse mount lenses that are currently in your possession. You can try the cheap but great quality Raynox DCR-250 filter. There are also bellows and enlarger lenses, microscope objectives, etc...
Hope this helps, if you have any other questions feel free to ask,
An example of bokeh of the Zeiss Makro-Planar T 100mm f/2.8