I also used a SL66 for years and own a Hasselblad 555ELD for half a year now. Basically, I can confirm everything that Martin already said. The SL66 is quite bulky in comparison to a Hasselblad 500CM and the protruding focussing knob adds to its dimensions. Lenses are basically the same, except for the newer lenses like 50 FLE, 60, 100, 180mm that were not available for the Rollei. If you only intend to use the classic lenses like 50mm, 80mm, 150mm, 250mm there is no difference, except that the Rollei lenses are much lighter and do not need any maintenance under normal circumstances, since they donīt have a shutter. By the way, I was stunned by the quality of the old 120/5,6 S-Planar. This lens is extremely sharp and has beautiful bokeh. As for helicoid vs rack focusing I think it is a standoff. The Rollei focusing mechanism is pretty smooth, but it can be worn out if the camera had seen heavy use. Regarding the Hasselblad lenses I only have experience with the newer CFi lenses and these are a real joy to focus. On the other hand Iīve heard that the older lenses can be stiff. As for the screens: I got rid of the Rolleiīs original screen soon after I bought it and installed the latest Rollei High-D Screen. What a difference! I think it is even superior to the much praised Hasselblad Acute Matte D, which I have in the 555ELD.
Please also mind that the Rolleis are between 30 and 50 years old now. It may be difficult to find one that is still in good working order, so you should plan in some money for an overhaul. I had a complete overhaul done on mine and it really made a difference. However, if you buy 30 or 40 years old Hasselblad equipment it may benefit from an overhaul either.