how good is the canon markII Think to switch from nikon reason are

Nikon D600 is better than 5D2 in every single characteristic.

Oil spots are annoying, but this is not the end of the world. You either send it back and get new one, or send it to service center for cleaning, or clean by yourself.

From time point of view, you will spend much more time with noise reduction on 5D2, than cleaning of dust/oil D600. Or if you need big DR, you will spend much more time on stitching HDR images, instead of just pulling shadows on D600.

And again, only a small part of D600 have oil/dust troubles . Nikon somehow has solved the problem, recent batches has no that issue. So I agree, I would not switch to 5D2 just because of that problem.