I prefer the optics of the RF.
The SF is a top quality roof binocular, but the field flattening isn't ideal. While the SF maintains better edge sharpness than the RF, and the field flattening reduces overall distortion. The problem is there is some remaining distortion, which I describe as complicated. You can read about it in my review of the SF:
Review: Zeiss Victory SF 10x42 - Rokslide
The RF is not field flattened and doesn't have perfect edge sharpness, but it degrades very near the edge and sort of smoothly transitions, which combined with the minor pincushion distortion makes for a very natural viewing experience. The image is similar to the Swarovski SLC or Zeiss HT.