Zeiss left side - Nikon right.
Well, it depends on what you're shooting. For these kind of shots I would suggest returning the Ottus immediately, it's really not worth it.
But, a couple of things are immediately striking, in addition to those already mentioned:
-despite de difference in magnification, which should give a serious advantage to the Nikon lens (see how bigger the door handle appears to be in the right image vs. the one on the left), the Zeiss image appears quite a bit sharper
-again, despite the difference in magnification, the Zeiss image retains a lot more details and tonal gradations both in the highlights and the shadows (especially in the highlights, in this case)
If these plus the other differences mentioned, from these quite mundane images, can be translated into something more substantial into your kind of work, by all means do keep the Ottus - there are very few (if any ! ) lenses who can deliver what this lens can deliver. But the objective and subjective aesthetic requirements of your own work have to provide the correct answers, not someone else on a photography forum.
For the type of images you have just shown, you can surely get by with smaller sensor formats and way cheaper lenses.