If I were you, I'd consider keeping the camera you have and upgrading the lens. If the camera is reasonably light tight and sound, there is really no reason to replace it. Go for the lens(es), a nice carbon tripod and ball head or a nice bag/backpack. That said, there is something rather nice about a new camera (:
Now to help answer the question you asked.....(: I do have a Tachi and like it a lot....light and easy to use. I've played with the Shen and it is heavier, but can take roll film backs. I think it may have some additional back movements, too. For me, it was not worth the extra weight.