I have both, the 15 and the new VC 75mm. They are so different in many aspects, that I can not see, how one can choose one over the other. I really like the 75 for portraits (but preferably on the M9) and I like the 15 on the R-D1 for landscapes. But I also use both on both cameras, but always for totally different subjects.
I would say, in your case, you absolutely need both of them! 😉