I have an RB67 with both 65mm and 50mm. For me, 65mm is ideal, it's similar to a 35mm lens on 35mm film which is perfect for my general purpose shots (I always found the 90mm on my RB67 just a little too narrow in field of view). The 50mm is pretty wide, a little too wide for most shots for me, though gives great perspective when you need it. To summarise, both focal lengths are great but I get more use out of 65mm, it's my general purpose lens if I have to take out just one focal length.