I believe you need to ask yourself how you intend to use the lens -- inside with natural light or outside; with family and freinds or candids of strangers, what other focal lengths do you have or intend to obtain, do you intend to take head and shoulders or half body shots, what is your main focal length, ect. The answer to these types of questions will give you some idea of the focal length needed, as well as the speed.
A very good lens, which can be picked up on the used market fairly economically, is the current 90mm Elmarit. At a max. aperture of 2.8, it will be somewhat limited in available light situations, which is the forte of rangefinder photography, but its optical performance is excellent; much better in my opinion of Leica's prior 90mm lenses, which tend to be soft wide open. As a practical matter, you probably will not want a larger aperature on a 90mm lens used for portraits for depth of field reasons.
The field of view of a 75mm lens is not much different then that of a 50mm lens. So if you have a 50mm lens, then the 75mm lens, in a sense, is somewhat duplicative.
The 75mm lens, however, makes a good companion to a 35mm lens. But so does a 90mm lens. I'm starting to believe that a 75mm lens would be a great indoor portrait lens, especially for friends and family where you can get as close as you want.
There is a cost consideration also: Leica 75mm lenses -- whether a Summilux or Summicron -- and, whether new or used, will be very expensive. Leica 90mm lenses can be obtained relatively cheaply.
The 75mm lenses are all (they are only two) optically great, even wide open. The same cannot be said for the 90mm lenses, except the current generations. But some degree of softness is not bad for portraits.
