I have both but have really not made any comparisons with the Sonnar. I suppose I could change my mind at some point but I seriously doubt it will ever replace my Sonnar. That lens is just special, and though it takes extra effort to capture the beauty, it is worth it.
To be honest the Nokton reminds me more of my Summilux and though it is certainly very, very nice, I doubt it will replace that lens either.