I have a combination of all 3 back versions for 120 that work on my ETRS so I'd not lose sleep on which to get. I've never screwed up anything using the older backs. They changed from the single to dual latch backs supposively because from time to time someone complained they inadvertantly opened the back if it had a single release. I'm not sure how as thye are not easy to open.
I have a pair of 35 backs and with them have eliminated the need for a 35mm slr system. Using the lenses that cover the 645 area means no light falloff and sharp as the center of the lens is all that is used. I also only need to know the charactor of the Bronica lenses. And, the Bronica controls. I do keep the screen with the 35mm lines in the camera all the time. Only downside is the AE II meter is metering outside the film area so some care needed when metering but as I use a handheld most of the time, it is a lesser concern for me. With the waist level finder only, the assembly is no heavier than an average SLR and with the metered prism and speed grip not more than many of the totl SLRs. When I load the 35mm backs, I usually carry the 40mm and either 75 or 105mm lenses. The 105 also is a macro so I have the bases pretty much covered. For portraits, I use a doubler on the 75 and the non-metered prism.
Why carry 2 kits when 1 will do?