Basically there is no difference between Preserve Input Format and exporting to DNG?
Preserve Input Format is a convenient way of saving your file in the same format that your original image was opened in Photo AI with
For instance, you opened your image as a DNG Photo AI will Export that image a DNG likewise, if opened your image as a TIFF file Photo AI will Export your image as a TIFF
Exporting to DNG you have the choice of choosing your file format
I guess in the case of Nikon NEF raw images Photo AI would not be able to save as the input format. It’s not a big deal, I get good results using either option.
I’m afraid not Photo AI can only save RAW files in the open source DNG format
Nikon as with Canon, SONY and other camera manufacturers like to keep their Proprietary RAW formats close to their chest a secret if you like
Therefore, company’s such as, Adobe have to use Reverse Engineering to make their own version of those RAW files
Basically the camera manufacturers won’t give up their proprietary information therefore, DNG’s are close copy’s to their formats but, will never be exactly the same as the camera manufacturers own Proprietary Format
Any edits done on a RAW file will change the format to DNG.
The only reason for using DNG itself instead of the Preserve format is to have access to compression options. If you enable/disable compression, then save, keep in mind this will become the new default for all next Preserve Format. Compression is enabled by default!