The 35mm (Full Frame DSLRs) are no more or less "mechanical" than digital MF backs. They do the same thing the same way, but with larger sensors, and until lately, the MF digital had/have a very limited ISO spectrum. I think maybe what you should be asking yourself is what you want contrast transitions to look like, detail from a larger sensor, and speed you want to shoot (that's ISO speed). You are already aware that the MF digital cameras (not the backs) are larger, cumbersome, and not as robust as the 35mm cameras. However MF offers a way to keep your film and have your digital too, since many take both backs. Another option is that if you want to keep your film cameras, buy a nice scanner and just scan the negatives. Then you can have your digital and keep your film too.