hi,
you have got the top in terms of body and optics, so the ideal would be to stay with Leica, their 35mm are without a shadow of a doubt a generation ahead. I mean they have no flaw. Contrast, resolution, subtle tones, flare resistance, flatness of the field, even from center to corners, and value for money (yes, really). I have used voigtlander's (1.2 and 1.4) and left them, did not suit my needs (especially bokeh-wise, terrible).
Sure...Leica's come at a price...unless second hand.
However, first of all, probably best is to follow the previous advices, see what kind of picture you like to take, in what conditions (ex: low light), and that will guide you to decide which is best for your needs.
Both voigtlander and Leica lenses have their own character; I personnally use different lenses depending on the body (film/digital), the subject, the conditions, and/or the type of film.
Possibly, a cheap option is the new summarit 35mm 2.5. It is a better performer than the summicron IV, but still follows the same optical design which will guarantee you the legendary leica feel of the summicron IV. It opens only at f/2.5 but that's not far from f/2, and with B&W film you have all the required latitude to compensate.