Sanj, I have found this to be the case with my Zeiss 21mm f/2.8, an incredible lens, which I bought for its true superiority over the Canon, 16-35mm f/2.8L II, in both sharpness and contrast. ...BUT after selling my Canon zoom and purchasing the new Canon 16-35mm f/4L IS, I have the same dilemma as you...the image quality from the new Canon zoom is so close to my Zeiss that I (horror of horrors! LOL), am considering selling the Zeiss. Same deal as you...the f/2.8 aperture just isn't enough that I find myself using it, and the IS and the AF on the Canon are making it my lens of choice.
There is that other reason not to sell the Zeiss, though....I get such real enjoyment out of just holding it and lustily coveting it.
The Canon pales in comparison there!
Sanj...I also notice...eh hem....that you are holding on to your Zeiss 135mm f/2.0 which (according to Dustin Abbott's timely review), blows away the Canon counterpart for sharpness and micro contrast. Very wise of you...as that Zeiss still has practical AND coveting qualities that are second to none!
So...with that in mind it should be easier for you to pry your little fingers off of your 15mm and 35mm Zeiss's and let someone else have a little coveting time, at a discount...c'mon...you can do it...just close you eyes and hold you 135mm...that's a boy.......