Well, don't know if I did it all wrong, but skiped the 50, and got a 35-90 two lens set-up to go along with my M2 (2.8 summaron and 2.8 elmarit). If I would miss the 5cm, I could always use the elmar on my III-f.
Answering to your question, and provided that your camera will accept the 28 with no extra gadgets, I choose 50 + 28 because of the difference in focal length.