Another vote for the 75 lux. I like the 35/75 pair for a minimal kit. Since you already have a 50, maybe the 90 makes more sense. I really like the last generation pre-ASPH Summicron, but it is quite big. I tried the Tele-Elmarit but found it far too sensitive to flare. The Elmarit-M and Summarit-M I have never tried, but both seem very nice. I would probably go with the Elmarit, mostly because of the built-in extending hood. My current choice for 90mm is the C-Elmar (f/4), or actually the Minolta M-Rokkor (which is supposed to be identical). Haven't used it much on the M9 yet though, but it is small and light weight, and probably a good choice if f/4 is enough and size/weight is an issue. Cheap too.
Edit: Just a note on the Voigtländer Color Heliar 75/2.5, which I also have. It is a great lens, but the close focus limit of 0.9m makes it less than optimal for portrait use on the M9, imho. The 75mm lux/cron both focus to 0.7m, which is very useful. I believe all the 90mm lenses go to 1m, which is ok.