The Df and D4 RAW files look fantastic at those ISO settings, whether or not it’s needed is going to depend on what you shoot. Concert photography, events, really any available light work the extra expense for the Df is going to be well worth it. But of course the D610 is an extremely capable all-around shooter, no doubt.
Dual SDs, especially since the Df is not a video camera, are not such a big deal as fast cards have never been cheaper. I have yet to fill the 32 GB primary card in my D800 shooting RAW stills with some video.
Started out doing photography at the age of 6 using an uncle's old 1940 kodak brownie box camera. At 15 years of age, I decided to buy my very own 1975 Praktica SLR camera. I now shoot with a Nikon D850. I do unpaid TFP and commercial paid work.