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I agree the two calls should probably be consistent, though I also suspect that VOLUME_NAME_NT is always longer than VOLUME_NAME_DOS.
My justification for using VOLUME_NAME_NT is that the final name might not be located on a DOS-accessible name. My suspicion was that VOLUME_NAME_NT was more general purpose (though somewhat less user-friendly), so preferable in the case that the name isn't exposed to the user. It's been a while since I've investigated it, but I think you can have symlinks to UNC paths as well as \\?\ paths, so my concern is VOLUME_NAME_DOS might not work with those paths. Some tests are probably called for.
In the case of nt._getfinalpathname(), it's not obvious to me how it might be used, so VOLUME_NAME_DOS may be more appropriate. Or perhaps _getfinalpathname should pass that parameter through.