Upload & Sell: Off
How much was the Zeiss 100?
New, the ZF.2 version is ~$1850. I picked up a ZF version (no chip) used for $1200. I think the ZF version is still manufactured, but I haven't found one available new, and I have no idea what the price would be.
If you had to keep only one, which would it be?
Ha! THAT is dooozy of a question. I LOVE this 135/2, and recently proclaimed it my favorite lens. But the 100mm focal length is generally better for me, especially for indoor portraits.
With the 135, I am using green dot focusing, and I am nailing my shots quickly and easily. Like 95% keepers. With the Zeiss, it's lower, but still reasonable. See my recent thread on the Alt Gear forum for info about that and some really great posts from others.
I also love the way the 135 feels. It's a hefty, beautiful, awesome hunk of glass. People go on about the build quality and beauty of Zeiss lenses, and they are indeed impressive. But I'd put my 1976-built 135 up against the new Zeiss in those respects any time.
I short, I don't know. If I could use the green dot, and nail my shots with the Zeiss with the same frequency, I think the Nikkor would go. Fortunately, it's a decision I don't have to make.