i think the hassey and the sl66 are both perfect camera's.. after seeing the dvd of brett weston i became interested in the rollei sl66 , i think when you do a lot of macro photo's or want to use petzval lenses as gandolfi the sl66 is a unique camera
though i recently choose for a hasselblad 501cm, reason; a much more brilliant viewfinder by the acute matte D focusing screen and the camera is lighter
If yo do a lot of work with flash, then the Hasselblad might be the better choice because of the ability to synch at any speed due to the leaf shutters in the lenses. The Rollei has a focal plane shutter with a flash synch speed of 1/30, which many have said is inadequate. Rollei did have one 80mm lens with a leaf shutter.
Otherwise, the lens designs were identical to the comparable offerings of that era, because these were Carl Zeiss lens designs and not Rollei.
I have the older SL66 lenses from the 1960s and find them to be excellent in terms of contrast, resolution and color reproduction.
The body is larger, and if you want to do any handheld shooting, then you want the bracket and the cable release. It makes the camera very manageable.
The SL66 also has depth of field preview and mirror lockup. The backs can handle either 120 or 200 merely by flipping a lever on the back. And it's simple to tell if your back is loaded.
It's a lovely system, and the bellows give you a large amount of flexibility.