Rachmaninoff is much more common. Martha Argerich spells it 'ff' on her 3rd concerto cd. If it matters, so does Horowitz, at least on the 1978 recording. Oddly enough, Argerich's recording of the piano suites with Rabinovitch spells it 'v'. My personal favorite Rachmaninoff recording, the Slatkin orchestral works, spells it 'ff'. Emanuel Ax's recordings always spell it 'ff.' Orozco's concerto recordings spell if 'ff'; Zimmerman spells it 'ov'. All of my printed music uses the 'ff' spelling.
By habit I prefer 'Rachmaninoff'.