I really don't get why any time Ken Rockwell is mentioned a huge mass of hate is vomitated against him here. It's really revolting, and the spectacle of people who doesn't even know what's going on but just jump on the insult bandwagon is really disgusting.
I remember Ken Rockwell's site among the first giving any kind of advice about lenses' quality, when the internet was still at the beginning and there was not much information at all, especially about lenses. He's a nice guy and his reviews are usually amusing and informative. He has his own tastes and requirements, just like all of us, so I don't necessarily agree with
every conclusion that he draws, but again I see no reason to hate him or despise him for this. We can also argue if his way of testing lenses and/or his whole website may be considered surpassed now, but again that's no good reason to hate him, and on the other hand I seriously doubt that more than 1% of the subscribers here would be really able to read an MTF chart.
While people was vomiting hate against Ken Rockwell, user
Dismayed linked to a very interesting website, which of course was completely ignored by the haters. A side-by-side comparison between the Nikkor 50mm f:1.4 and Zeiss Planar 50mm F:1.4 is quite interesting and tells us that the Zeiss has slightly more distortion, but in turn is more sharp at center especially at mid apertures, but again in turn the Nikkor is more equally sharp at corners, but once again the Planar helds sharpness a bit better at closer apertures. So we end once again (surprise surprise!) with the classic
adagio: you gain some, you loose some. None of the two wins hands down.
Buy used at a fair price. Test it yourself and if you find that you don't like it for some reason then sell it. You will get most of your money back and any bit you lose just consider a cheap rental fee.
One big difference between the Zeiss lenses and the Nikkors is that the Zeiss are manual focus only. If your eyes are not what they used to be then you may prefer autofocus. It doesn't matter how sharp a lens is if you miss focus it won't be sharp.
Alan has a good point here: if you need/want autofocus, there is basically no question. (The point in which he says to buy both and resell - that's instead just for wealthy people for which a Zeiss lens is just a small fraction of his/her salary; by the way I'm having a very hard time in reselling photo gear that I don't need anymore).
I would also add that Zeiss lenses usually have a high quality 9-blades nearly-circular diaphragm, while Nikkors typically have 7 or less. This makes a
HUGE difference in how defocused point-linghts are rendered on the image. If you like to see heptagons all over your picture, as it was almost "cool" in the '70s and the '80s, the Nikkor will be your obvious choice. If you consider a polygon appearing on the image anywhere a
defect of the image rather than a "cool effect", than you might seriously want to reconsider the Zeiss.