If you love the Yashica, I'd keep it. As for f/2.8, only you know if you need that, I know I don't on my Rolleiflex 2.8GX, f/3.5 would be just fine. With interchangeable backs, well it depends on how quick you need to change film types or reload. I used to have a Hasselblad with a couple of backs, it's cool, but I don't miss those backs now that I'm using a Rolleiflex.